I have noticed that almost every time new work is pushed I get at least 1 and sometimes 2 rejected shares. Over the last hour or so, I am getting two rejected shares every single time phoenix outputs "LP: New work pushed".
Also, that is literally the only time I get rejected shares.
Thats normal, your miner is in the middle of a getwork when the LP comes in and sometimes you will find a share at or around the same time as the LP, causing you to submit it and it becomes stale. The lower your latency to the pool the less of these forms of stales you will get (and to a greater extent the better peered the bitcoind is behind the pool).