Hi,
There are no rule of thumb as such,If you have Sanmina DFM Pdf file,you query is been answered there.Since most of the company doesn't follow any rules,they just standardize it for their inhouse operation after going through lots of iteration before product release.Finally they make that set of rules as guidelines for their own use only.
Bottom line, if you go thru any Dfm/DfA Document there are guidelines for your subject,but then its not that you can deviate from that,its just a reference.
Better you call up your local Pcb Assembly, they might help you in this regard and also give you precise information.
Regards
Ramesh