No, Slush is score based.  Shares contributed later in a given block are scored higher than shares contributed in the early portions;  this is meant to discourage miners who join for the first, lesser difficulty portion of a block, then jump out to somewhere else when the CDF crosses whatever threshhold they've determined is "too hard too long" for them (See also:  Raulo's attack, https://bitcointalk.org/index.php?topic=3165.msg44357#msg44357 ).  The block is then scored as a whole, and rewards are based on the portion of the score of the block that you accumulated.