If you're using GearShift (for HDV files), you might run into a sync issue, where the audio for individual clips was offset by a number of frames. I don't know if it was a GearShift issue or a Vegas version issue.
Jeff,
I've had no problems opening V6 projects in V7. However, if you think you'll need to go back to V6, make sure you save the V7 version with a different name.
If your outstanding projects are for real money, you can try, but expect something to act different. I would never switch tools in the middle of a project with a deadline unless a)it is an add-on tool or b)existing toolset is broken beyond repair.
And definitely keep version 6 installed for emegencies.
We, as an experienced group, cannot stress strongly enough that you should always copy V6 project files to a different name before opening them with V7. Once you've saved a .veg file in a new version you can't ever open it in an older version again.