Well, the pushrods expand, but the valve stems (especially the hotter exhaust valves) also expand. And the block height is growing and you have to remember the rocker arm ratio too, so the pushrod growing doesn't mean you can just add the amount of it's growth to the valve clearance number. For mechanical lifters (solids), it's better to just add a few thousands cold or set them "fat" or "heavy duty" as grigg said, and then do the final adjustments when everything is warm. Hydraulic lifters are adjusted totally different. Those you would lash to zero clearance and then slowly crank a 1/2 turn or so into them. The hydraulic cushion will take up for any slight misadjustment from there.
