As I recall, the time taken to beat the levels, including the guide the red truck levels, are saved, right from the very first time you do it. However, what I found to be the best, is clear the path, exit the level once that was done, then go back into the level to do all of its side objectives, (destroy all buildings, collect all rdu's, if applicable, find the scientist, also if applicable, find all the level unlock satellites.)
Because the times it you to clear the level does stick, if you manage to get a platinum time, you will still get gold, until the time you start earning platinums, however, at that point, any levels with platinum times automatically get awarded a platinum metal, meaning you don't have to go and beat the level again.
Finally, what I remember, is clear the easy levels, by clearing the path. Beat the medium levels, beat the hard levels. FInd all the scientists, clear path for space shuttle, play through the moon and clear it at least once. Get gold on every level you have, including levels you may have yet to find the unlock satellites for. The remaining planets get unlocked, get gold on all of them. Once that was done, get platinum on everything.
As far as things go, I never did get platinums on everything.
