I have a problem like that with an RG 250 DX I have. Its like mikejcurtis said, the problem is with the material the bridge is made of. The bridge just doesn't move as smoothly as it should against the post thingies, and the trem doesn't come back to rest exactly where it started out from.
I was gonna change it to a better model trem, like an Edge or something, (currently its an Edge Pro II) but I figure I'll just buy a better guitar with a better trem
someday.
Anywhoo, for now, finesse is how I deal with it. If you find the bridge isn't comming back to where it should be, use your ears and playing ability to correct it. Oh yeah, thats also kinda tricky. ;) Maybe I'll look at this trem setter thing.....