Previously, I did all of my editing with DV source video. I had no problems trimming clips and having them begin and end on the frames I wanted in the final video.

Now my source video is XviD encoded AVI, with an I-frame interval of about 10 or 300, depending on the source.  If I try to trim one of those clips to begin and/or end on a frame that is not an I-frame, then sometimes the final video doesn't begin and/or end of the frame that I thought it would.

Any suggestions?  I could re-encode the source video to DV before editing but I was hoping to get around that if I could.  I thought of re-encoding the XviD to have an I-frame interval of 1, would that work?

XviD and DivX have given me problems in the past with the same issue you have and also a jittery playback.  I have never had an issue with DV source files and have always found them incident-free when it came to editing them.  They are large files but who cares as you will most likely render them to another format anyway upon completion. 

I spent a lot of time and tears trying to make those two formats work and quite frankly gave up on them.  IMO save yourself some grief and just convert them.