No, if you export from Avid with the 601/709 button selected, you are telling the export module that the source is already 709, so it leaves it untouched.
If you export with the RGB button selected, it will stretch 16-235 over 0-255, clipping of any whites over 235. Not a good idea if you need to grade indeed.
Not sure what you are comparing and how the footage has been brought in. HDV is usually captured digitally (FW or HDSDI) and that should leave all levels untouched during capture. If you export same-as-source and 709, you are basically not changing anything, leaving the original intact.
Are you sure you are not being bitten by some gamma issues within QuickTime?