I'm a relative newbie, but my own solution to the thing I think both you and I came up against (changing margins for one tag may sort out spacing in one part of a site, only to disturb a previously good spacing elsewhere?) is now to allocate an equal margin top and bottom for all tags. You might well want more space above and below an H1 than for an H3 or a p, but in general I think pages look good with equal spaces above and below any given piece of text, so this solution works well for me and meets your 'simple rule of thumb' spec too, hopefully!
