Many of the mods / hacks I've seen use low precision resistors which will require you to poke around until you find what the servo thinks is center-position. That approach will require you to capture that value and use it as an offset in your software... which makes re-using your servos much more difficult. Using high-precision resistors is the way to go.
Cracking off a piece of the potentiometer housing is a quick and easy way to remove the servos rotational limits. It's also a great way to make the servo wobbly and grind it's gears; there is a better way.
Matched resistors give you sweet and smooth variable speed coming or going.
I guess you could just buy continuous rotation servos - but why buy when you can make!