I have a firm, rubbery mass on the right side of my sacrum. it is about the size of my thumb - painless - and somewhat movable. should i see a doctor?

Lipoma?: Sounds like it is a fatty tumor - a lipoma - although it is really impossible to know without cutting it out. "Bad" things are usually very firm, don't move and get progressively larger. If you find any of those things see your doc. Otherwise I think waiting and watching is the best strategy.

Probably a lipoma...: Your description fits that of a lipoma (a benign tumor), but let your doctor see it. If he or she thinks it necessary he or she can biopsy it or refer you to a dermatologist.

Eventually but...: I have one of those too (same location), but it is more the size of a peanut. This is almost certainly a benign growth like a lipoma and does not need treatment. If it enlarges or becomes painful then seek attention sooner, otherwise I think it is reasonable to wait until you are seeing a doctor for some other reason and you can show it to him or her then.