Bone marrow biopsy...or not...

My white blood cell count has been low for a long time. I have no symptoms of illness so the doctors aren't sure what's going on. I went to a hemotologist in Portland last September, she did several tests and found my B12 level was very low as well. She said that a low B12 level can also lower your white count, so I've been receiving B12 injections monthly. My follow up blood test still showed by white count low, but my B12 level is now normal. The hemotologist called and said my next move is up to me. I can do a bone marrow biopsy to see if we can find out why my count is low, but sometimes the biopsy is inconclusive. Or I can continue to have my white count checked every 3 months and if it goes any lower, or if I get sick or get any kind of infection to call her.
So I'm not sure what I should do....bone marrow biopsy...or not?