Do I need an accurate clock for an accelerometer?
It sounds like your measurements are already on an even interval controlled by your device, so you could simply add a serial number to each measurement or block of measurements to ensure that you don't lose one. This could be as simple as a single byte, which would have lower overhead than a full time stamp.

While a dedicated RTC could provide very accurate time stamps, it would not necessarily make the timing of your measurements any more accurate unless you drive those samples off an interrupt from the RTC. What matters from a filtering standpoint is how low the jitter is on the sampling of the signal, not on receiving the data.