I've watched a video of a guy rerolling 3 legendary skins and getting a 520rp skin. So yea, as the others have said, it's not worth rerolling legendaries. If you don't want them, disenchant them for the massive amount of essence. You'll need the essence eventually to upgrade a shard you want in to a permanent skin!
I think rerolling anything but 520s is probably a poor decision. I tried rerolling two 520s and a 750, and got a 520 skin for a champion I don't even own. Was pretty disappointed and immediately thought it was a mistake because while I didn't want any of the skins I rerolled I'd have at least used them (e.g. I never play fiddlesticks, but I own him and if I got him in an ARAM I'd have used pumpkinhead I guess).
One day you'll get a shard for a skin you REALLY want and won't be able to afford to forge it :P So just sit on them until you actually have the shard you want to disenchant is my best idea. There doesn't appear to be a limit to the number of shards you can store.