Valve lash can be used as a tuning tool on the engine. Cranking down on the lash a few thousanths makes the cam appear larger to the engine, it adds duration and obviously a very small amount of lift. Opening up the lash has the opposite effect.
Depending on the design of the camshaft some engines can be rather sensitive to changes in lash. Most of the time messing around on the dyno I change it about .004 one way or the other to see if I can find anything, sometimes it helps, sometimes not you never know, either way, it allows you to custom taylor the camshaft to the engine, to make it more optimum and often times it will show you that you need a slightly different cam either larger or smaller than what's in the engine.
Loose lashing is more dangerous than tightening because it can increase the shock loads to the valvetrain again depending on the design of the lobe. Some tight lash designs can get rough very quickly while others are much easier and will tollerate much more lash than what's on the card without problems.
For instance, the Reed cams GTL lobes are a tight lash series that I've used a lot in circle track Chevy engines. I've run those lobes for extended periods as tight as .006 on an intake valve and as loose as .018 on an exhaust or intake!
I would recommend consultation with the cam grinder to see what lash the lobe will tolerate without breaking up the valvetrain or hanging a valve. They should be able to tell you what will work ok.
Some SuperStock racers run a very large cam and then just lash it till it runs it's best, whatever it takes, .050! That sure gives the cam some action and it's very rough on parts, but if the races are short and the parts are good they live for a little while.
