I haven't used the trim feature of MAX. The User guide suggests the following on pg. 61:

Select Trim Until End to Trim the track all the way to the end of the song.
Select Don't Trim and the levels of the tracks will snap back to their previous levels at the end of the Trim.
Whatever is chosen, if No More Warnings is checked, then the current Trim to End setting will be saved in the Preferences and the dialogue will not appear again.

Trim Until End Preferences
In the Preferences Dialogue in the Edit Menu, under "Trim Until End", check either Always, Never, or Ask First.

When Always is selected, the Trim Until End dialogue will not appear at the end of the pass. The software Trims the track for the entire mix.
When Never is selected, the Trim Until End dialogue will not appear at the end of the pass. For every pass where a Trim is performed, the Trimmed tracks snap back to their original levels after the Trim.
(You always have the option of course of turning the Trim dialogue back on again by clicking Ask First in the Preferences dialogue.)
When Ask First is selected, the Trim Until End dialogue will appear at the end of the pass as just described, giving you the option with each pass to either Trim until End or Don't Trim.