I've been a plumber for quite a while, but have limited experience with boilers. Mostly I just pipe them up according to the design, so keep that in mind when I say this; I thought it was commonly accepted that "pumping away" (there's an article on this by the same name) from a boiler was the preferred way, as it kept turbulence inside the boiler and the various devices, (low water limit, etc) located near the boiler, to a minimum.
