Hi Thad:

Actually there are a lot of elements to consider including size, thickness and glass type. While you may want to leave room for a fan, I would not reccomend installing one until you have confirmed that the conditions under typical use warrant one.

I prefer to stay as far away from moving air as possible. This just asks for dust and a fan moving air generates extra static that will cause the metalized coating to suck dust. My 22" truss never seemed to need a fan while smaller solid tube scopes did need some venting at least at the start of viewing. I too would prefer to pull out but because the air around the feet is more likely to be disturbed and dusty. The edges normally cooly faster than the center and pulling out will increase this effect. Probably still Ok with full thickness PYREX or better but thin PPG may require you to blow in. Just flip the fan and experiment would be my suggestion.

Good Luck
-Rick