There's also a significant difference between tightening the cranks on a road bike and on a fixie, due to the different stresses involved.

If you're using a fixie on the street, you're going to be putting substantial pressure on the crank in both directions, so you're going to need to tighten the crank bolts a lot more often, especially if you're a heavier rider.

Don't crank them down so tight you split the crank, but do tighten them back up after every ride or two.