Some books strongly encourage covering the front brake w/ 2 fingers at all times, including on curves... but they are talking about performance (competitive) riding where the brakes are used whenever needed including deep in curves, not MSF riding where the brakes are used only when the bike is going in a straight line.
Other (less performance-oriented) books say don't cover but use 2 fingers on the front brakes unless you are riding a pre-99 harley or other bike with marginal front brakes, in which case you'll need all four fingers on the brake.
MSF basic rider course drills 4-fingers and not covering the front brake, but I suspect part of the reason is that they want to be able to judge when you reach for the brake and can't do that if your hand is already there.
When I rode bicycles (mountain and BMX) a lot, I always covered the brakes with two fingers. The brake levers were designed for two finger use, and it was frequently necessary to hit the brakes quickly.... I was fighting those habits all through the MSF course....and, free from the baleful glare of RiderCoaches, I'll go back to two fingering the front brake but not really cover it.
BTW: I made a point of skidding the front and rear tires during the basic rider course... the rear quickly fishtailed and I rode it to a stop. Locking the front felt about the same on a motorcycle as on a bicycle... you haven't really lost control, but the only thing balancing the bike is you and it tells you in no uncertain terms, "I *will* go down if you aren't quick."
