Good questions. MisterX were you thinking of just pulling from the rails? I'm not sure that you meant this, but it would burn a lot of wasted power. OTOH, we could consider a switching regulator (as in the CTH) which is much more efficient.
Beefy I designed a 12.6V supply because I thought you would only have one and you would put the tubes in series. It is easer to make a regulated supply at 12.6V than it is at 6.3V from off the shelf transformers. A 12.6V transformer yields a high enough DC that at 350mA the regulator won't drop out. And, to be sure, I figured we would use an LDO regulator. This is all, as you know, very common.
A simple rectified/filtered DC supply usually has problems with line regulation and almost always requires dropping resistors and yields a variable heater voltage depending on the line voltage. Whereas with a 12.6V regulated supply you get rock steady heater voltages.
This was my reasoning, but we're still designing so we can go wherever there is consensus. I was just thinking that if one board had the rail PS and other the heater PS you would get everything you need with just two snappable boards.
Edit: I see that jdklake's post squeezed in before mine.
Edited by runeight - 8/19/10 at 6:14pm