That's not really abusing the score system, since doing that you will actually end up with less coins than if you just mined on Slush 100% of the time. When you're pool hopping you want to mine on the proportional pool (or Slush's Pool) at the very start of a round, and then hop away after a certain period of time if a block hasn't been found.
I thought with slush's pool, if the block is long and you only contributed in the first portion of the block and you leave elsewhere, your score/shares become becomes useless.
You do. However, you're less likely to solve a block in a given amount of time near the end of the block than at the start. ooc has a good blog post on slush's pool from quite awhile ago that is good reading.