Really the only rule that I tend to stick to is trading Heritage sp for Heritage sp. Personally I never ask a trader to trade Heritage sp's unless I have a Heritage sp to trade back.
On a very positive note, because TCDB is so easy to keep track of a collection, I have a lot less Brewers commons than when I traded on other websites. Partly for lack of time, I would tell someone to just send me a few Brewers cards in return. Since trading on here I have built my collection nicely because it is a lot less work to get the cards I want or need.