I was worried about this after surgery and sent my husband off to get an electric shaver. Then I had chemo and hair was not an issue for a while! I also had radiation to that area and between that and the tamoxifen (causes hair thinning) I've barely needed to use anything under that arm! I did find that getting an electric shaver with a trimming/cutting edge was easier to use under the arm.
I've never cut myself shaving under the arm but I guess now that it is more important to not injure this area that would be just when it would happen!
It can be hard to shave under the arm at first because of the restricted movement from the op, but it does get better with exercise. Take care. Deanne xxx