Are you going to use the engine for racing?, if not, leave it be. The karting guys push every allowable rule and when they run out of options, they move to the next thing that isn't in the rule book to gain the smallest edge. A human hair is approximately .004", most aftermarket cams recommend setting your lash at .003", even if it did grow when it got hot, what's it going to grow to?, .004"-.005"?. In the grand scheme of things, how much time does it save the engine to open the valve .003"?, not to mention setting zero lash isn't the easiest thing to do. Just setting it to where you can still wiggle the rocker doesn't mean it's at zero lash. To set zero lash requires a good sealing valve job and the ability to pressure check the intake/exhaust port for bleed down. When considering modifying anything, you have to ask yourself "is the gain worth the risk", "good enough" usually results in burnt valves or lost power thus the gain really isn't worth the risk.