As a cop I have real trouble seeing why some places feel the need to specifically outlaw lane splitting. Even if I don't think like a traffic cop, heartless bastiges they are, I can come up with three California violations that cover bad behavior by lane splitting bikes. Speed unsafe for conditions, unsafe lane change (no such thing as always staying in one lane when you are splitting) and changing lanes without signaling. Add them all together, mix in a bad attitude by the rider, throw in a touch of F you too buddy from the cop and you might even turn it into a reckless driving arrest (not a ticket, a genuine go to jail arrest).
As for where and when I split lanes.
Freeways w/regular lanes w/ heavy traffic below 15-20mph, split on!
Freeways w/carpool lanes separated from the regular lanes, just slightly slower than the motor cop in front of me (50, 60,70?)
City streets I'll filter (learned that term from Brit bike mags) up to the limit line at every light. Won't split moving traffic. Easier to wait for the light and slide up easy at the light.