I have a large number of clips that will be used first in a standard def DVD and possibily in HD sometime in the future. To save time I am tempted to render effects at standard quality for this first SD DVD pass and re-render at highest quality later if I need to output HD. I figure the quality difference will not be visible in SD output since it is an HD source to begin with. (?)

I am in your situation often, here is how I approach it.

I render in HD and do a "SD Downconvert" for the SD output.

In the HD render settings, I keep it set to "standard", which saves a lot of time (even in HD).

Thanks, I'll stick with the "standard" renders for this version... I'm going straight to DVD so instead of downconverting I'm keeping everything DNxHD and using QT refs for the SD MPEG-2 files.