This is quite a complex issue. When working with physical frames, say with 35mm film, frame rates are extremely important and mixing frame rates is going to become an audio nightmare unless it is very carefully planned in advance of filming and the production sound mixer and audio post team really know what they are doing, as it involves carefully manipulating the sample frequency of the audio both during filming and in post.
Working purely in digital for capture, editing and output can make life easier, in a sense, because converting between frame rates can be done with interpolation, rather than speeding up or slowing down fixed physical frames. However, there are numerous dangers with this workflow too. How exactly is the software interpolating, is it just interpolating the images or is it also interpolating the linked sound? If it's the latter, you're likely to run into a bunch of hurt. If it's just the image then the only worries are visual artefacts and any slight timing changes, which could easily affect audio sync.
Very recently I had a 1080p short film delivered to me for audio post and it had been rendered at 24fps by mistake, instead of at the original and delivery frame rate of 23.976fps. This caused all kinds of audio problems and was very difficult to diagnose because of course both these frame rates have 24 frames in a second of time-code. Had this problem not been detected, there's a pretty good chance that by the end of a 5 minute short, even with this very tiny difference in frame rates, the audio would have been out of sync with the picture by about 7 frames.
There are several different workflows to how we sync sound to picture, the best is from time-code embedded in the audio files during filming but this requires the recorder to be time-code sync'ed with the camera/s during filming. Our preliminary guide is from the work of the picture editor but this usually need adjustment as the editor does not have the equipment to accurately sync sound and picture. It's a big subject area to get into in a short post, so I won't! But just to say; if you can, always film with camera/recorder time-code locked and always slate, as the more info we have the better. I gave some info on workflow and materials exchange between picture editor and audio post here:
http://www.indietalk.com/showthread.php?t=41292
Bottom line is that changing or mixing frame rates is fraught with dangers, especially if handing off the sound to an audio post guy. You might be lucky and get away with it, depending on what your software does and how you use it but as a general rule, avoid it like the plague. As a cautionary tale, I was once involved in a professional project which ended up going bankrupt and never got finished because of frame rate issues. I've heard similar tales from others.
G