depends on the aircraft. i did the first half of my training in a cherokee 180, in which i used the preferred method of crabbing into the wind and kick straight just before touchdown. slipping was possible, but you had to straighten out before landing because of the nosewheel steering. cessnas have limits on slip time because of the aerodynamics of the aircraft (airflow over the tail, i think).
i currently fly a grumman cheetah (hypercheetah mod). since the grumman steers by differential braking (free castoring nosewheel), i can slip all the way down to landing and touchdown with the rudder at full deflection. this lets you pin the main on the windward side down and then transition to rolling as you loose rudder authority and gain braking authority.
heavy iron (airliners and such) HAVE to crab, as a slip cannot be maintained once they get close to the ground (wing mounted engines will touch down first). thus, boeing products have crosswind limits based on the amount of crab the main gear can straighten out without collapsing (with a safety factor). an exception to this is the B-52, which has main gear that can be pivoted sideways, allowing it to land with the nose pointing into the wind and rolling at a considerable angle to the runway. i have heard an interview where the pilot said he told the co-pilot to lean back so he could see the runway...
