I'm not sure how you can build pressure in the cooling system without air, or some other compressible gas. Water won't compress...you can put pressure on it, but you're doing it with gas propogation or mechanical means (and mechanical means does NOT reduce the volume of the water).
Generally, when I see pressure come up in the cooling system, shortly after start up and when load is applied, that is a sure sign that the head bolts/studs are inelastic. The heads will start to grow quickly, and the gas leakage past the fire ring stops quickly....but cooling system pressure takes a jump when read directly. Usually, I see the head bolts/studs overtorqued. This can be really ugly when running aluminum heads.
Back to the original question about the water tank....a little personal experience (more like personal screw-up, but HEY...we're all friends here); we built the cooling tank for the 797 Roadster out of some heavy stuff, with big strong baffles, and a 14 psi cap.
When the headgasket blew (after boost pressure got so high that the combustion chambers moved up and bent the head) the pressure in the tank resulted in "form fitting" it into the car. The problem is that a 14 psi cap can't flow as much out-gassing as a broken 2-liter turbo engine can make!!
Bobby Sights had to unravel that mess (sorry Bob) and if I had it to do over, I'd have fitted a real blow off valve (away from the driver area), in addition to the pressure cap. That way I could have chosen where the steam went, instead of into the cockpit.... and watching my driver turn in to the center of the course in '99 with what appeared to be smoke pouring out of the cockpit.
I'm real glad there was an extra panel of sheet metal between the tank area and the driver....but I sure could have done better, hindsight being 20-20. Anyway....leave some room in case you swell the tank...you may need to take it out without cutting tools!
And plan for the day that something blows and a hose or connection has to burst....you can decide where that should occur, in the system and away from the driver.
Regards, JimL