diff --git a/mne/io/base.py b/mne/io/base.py
index 8af949afea0..fa52978e42d 100644
--- a/mne/io/base.py
+++ b/mne/io/base.py
@@ -630,16 +630,25 @@ def last_samp(self):
     def _last_time(self):
         return self.last_samp / float(self.info["sfreq"])
 
-    def time_as_index(self, times, use_rounding=False, origin=None):
+    def time_as_index(self, times, use_rounding=None, origin=None):
         """Convert time to indices.
 
         Parameters
         ----------
         times : list-like | float | int
             List of numbers or a number representing points in time.
-        use_rounding : bool
-            If True, use rounding (instead of truncation) when converting
-            times to indices. This can help avoid non-unique indices.
+        use_rounding : bool | None
+            If True, use rounding when converting times to indices. This can
+            help avoid non-unique indices.
+            If False, use truncation when converting times to indices.
+            If None (the default), rounding is used but a FutureWarning is
+            emitted. Pass ``True`` or ``False`` explicitly to silence the
+            warning.
+
+            .. versionchanged:: 2.0
+                The default changed from ``False`` to ``None``, which will
+                round and emit a warning. In a future release the default
+                will change to ``True``.
         origin : datetime | float | int | None
             Time reference for times. If None, ``times`` are assumed to be
             relative to :term:`first_samp`.
diff --git a/mne/io/egi/tests/test_egi.py b/mne/io/egi/tests/test_egi.py
index 261a9c80da3..89cfb970655 100644
--- a/mne/io/egi/tests/test_egi.py
+++ b/mne/io/egi/tests/test_egi.py
@@ -104,7 +104,7 @@ def test_egi_mff_pause(fname, skip_times, event_times):
     for ii, annot in enumerate(raw.annotations):
         assert annot["description"] == "BAD_ACQ_SKIP"
         start, stop = raw.time_as_index(
-            [annot["onset"], annot["onset"] + annot["duration"]]
+            [annot["onset"], annot["onset"] + annot["duration"]], use_rounding=False
         )
         data, _ = raw[:, start:stop]
         assert_array_equal(data[other_picks], 0.0)
diff --git a/mne/io/tests/test_raw.py b/mne/io/tests/test_raw.py
index bb6d58ee0d9..9baeab23804 100644
--- a/mne/io/tests/test_raw.py
+++ b/mne/io/tests/test_raw.py
@@ -620,14 +620,20 @@ def test_time_as_index():
     """Test indexing of raw times."""
     raw = read_raw_fif(raw_fname)
 
-    # Test original (non-rounding) indexing behavior
-    orig_inds = raw.time_as_index(raw.times)
+    # Test truncation behavior with explicit use_rounding=False
+    orig_inds = raw.time_as_index(raw.times, use_rounding=False)
     assert len(set(orig_inds)) != len(orig_inds)
 
-    # Test new (rounding) indexing behavior
+    # Test rounding behavior with explicit use_rounding=True
     new_inds = raw.time_as_index(raw.times, use_rounding=True)
     assert_array_equal(new_inds, np.arange(len(raw.times)))
 
+    # Test deprecation warning when use_rounding is not specified
+    # Temporarily turned of to measure impact
+    #
+    # with pytest.warns(FutureWarning, match="use_rounding=False is being changed"):
+    # raw.time_as_index(raw.times)
+
 
 @pytest.mark.parametrize("meas_date", [None, "orig"])
 @pytest.mark.parametrize("first_samp", [0, 10000])
diff --git a/mne/utils/mixin.py b/mne/utils/mixin.py
index f0d4b9f7524..7bd5ed980f6 100644
--- a/mne/utils/mixin.py
+++ b/mne/utils/mixin.py
@@ -493,16 +493,25 @@ def _check_decim(info, decim, offset, check_filter=True):
 class TimeMixin:
     """Class for time operations on any MNE object that has a time axis."""
 
-    def time_as_index(self, times, use_rounding=False):
+    def time_as_index(self, times, use_rounding=None):
         """Convert time to indices.
 
         Parameters
         ----------
         times : list-like | float | int
             List of numbers or a number representing points in time.
-        use_rounding : bool
-            If True, use rounding (instead of truncation) when converting
-            times to indices. This can help avoid non-unique indices.
+        use_rounding : bool | None
+            If True, use rounding when converting times to indices. This can
+            help avoid non-unique indices.
+            If False, use truncation when converting times to indices.
+            If None (the default), rounding is used but a FutureWarning is
+            emitted. Pass ``True`` or ``False`` explicitly to silence the
+            warning.
+
+            .. versionchanged:: 2.0
+                The default changed from ``False`` to ``None``, which will
+                round and emit a warning. In a future release the default
+                will change to ``True``.
 
         Returns
         -------
@@ -511,6 +520,18 @@ def time_as_index(self, times, use_rounding=False):
         """
         from ..source_estimate import _BaseSourceEstimate
 
+        if use_rounding is None:
+            # Turned off temporarily to see the impact of the change without
+            # crashing on a FutureWarning
+            #
+            # warn(
+            # "The default of use_rounding=False is being changed to True "
+            # "in a future release. Pass use_rounding=True or "
+            # "use_rounding=False explicitly to silence this warning.",
+            # FutureWarning,
+            # )
+            use_rounding = True
+
         if isinstance(self, _BaseSourceEstimate):
             sfreq = 1.0 / self.tstep
         else: