As long as the valves are not held open when the engine is cold the valves can have less than .001 clearance. The pushrods are steel and the rest of the engine is aluminum. This means the cylinder will "grow" longer faster/more that the pushrods so the valve clearance will increase with heat.
With the above said I run my valves as tight as I can. I've had no issues to date doing this. My reasoning for doing this is to gain a slight advantage in performance. It's the motor head in me doing this.
Ken