I don't know if Seb has any specific recommendations for his gear, but generally you want to make sure that there is a bit of airflow around the back of your rack for all your gear. Make sure any vents on equipment are clear and unobstructed. Leave a 1U gap between any heavy heat producers such as power amps and other gear. It is probably best to separate preamps and gear like power amps anyway, because of the relative signal levels, a power amp will be electrically noisy while a mic pre is very sensitive.
If you have any really hot gear, leave enough of a gap between it and the gear above to fit in a few potatoes in foil, by the end of the session they should be done nicely! :D

Obviously it depends on the unit....i.e a four-channel vmp is going to kick out a lot more heat than a two channel...
For those of you with one channel vmp ,two channel vmp or THORAX ,, you needn't worry about excessive heat dissapation.
There's enough distance between heat producing components to radiate out....
However for four-channel vmp ( vmp-4000e ) or veqc units , it is suggested an inch or so clearance for air flow would help in maintaining performance levels and would preserve the unit's lifespan. 8-)
It's not a rule or anything ,... there are hundreds out there that have been going for years without any spacing at all....

Class A designs always tend to put out heat 'cause it's like having your car in idle even when you're not driving.... 8)

This is the only space i can think of actually ......
http://www.lpi.usra.edu/expmoon/Apollo8/A08_MP.PhotosFS.gif

I have not noticed any significant heat build up with my vmp 4000e ... I keep about an inch above the unit open ...

I figured there's that cool ~S~ logo cut into the top for a reason, so I left some space for the unit to vent .. but after using it now for over 18 months or so, I would feel very comfortable racking something directly on top of the 4000e without a second thought.