I guess it all depends on what you are working on. For every auto I have ever done, the 17" block has been plenty good.
For waves that may be larger than that, a flexible aluminum yardstick kept in good condition will reveal problems quite well. They can be placed perpendicular to a flat panel or layed down on a curved one. With a bit of learning and technique, a critical eye and decent light, large size waves can be detected and the high areas worked down by carefully concentrating sanding in that area, and rechecking often.
I've even been known to rub the yardstick against primer or filler, a bit of the aluminum will rub off and mark the high area, though of course this puts wear on the yardstick over time...