Generally, you can tighten them up as much as you want but making sure that the valve is closing. You probably don't want to go that far. However, loosening them is a bit different. The looser they get, the harder the cam slaps the lifter after the lash is taken-up. One of the cam manufactures recommends no more than 0.004" looser than spec for their agressive lobes, FWIW. I've run as loose as 0.034" on a 0.026" spec.on an old style cam. Ran well, but sounded like a tin can of marbles.