You can get lucky on occasion but clamps are the correct
and recommended method.

By the way... putting the screws in by hand is a tad easier
in some cases. This also cuts down on "spin-out" of screws.

Kreg reccomends the use of a drill with a clutch. My old Hitachi cordless
drill does not have a clutch but it is relatively low-powered (torque) and
has a nice tactile/responsive trigger; that setup works pretty well.. With
my Panasonic cordless drill, I *have to* use the clutch and that works well
too. My Bosch corded drill w/o clutch would not work well at all.

Yes I would imagine that screwing by hand could be preferable to the wrong
drill for the job, particularly in softwoods.

-Steve