Is it possible to manipulate the location of radiant quests by saving prior to speaking with the quest giver, and reloading if the location isnt to your liking? Or maybe a better question is, when exactly is radiant location determined?

I know this works with Delvin and Vex in the TG when getting the numerous quests to restore the guild but not sure about any others.

Sometimes this works, but many quest givers seem to never change where they send somone

^^^^
In other words, many quests aren't all that "radiant".
Saving and reloading should work but you will want to save outside of the cell the questgiver is in.
Keep in mind that some quests will only have a few possible locations.

This, I haven't had any luck when I suddenly realized I didn't like where I was being sent and the only save I could reload was with me already in the cell with the other person. Damn you Farkas and your crazy, persistent dream of sending me to Shimmermist Cave!