Re: griptape on bokken
I think you need to ask why you need tape. Jun made a great point that you may be using the bokken inappropriately it you feel you need tape to maintain a grip on your weapon. Regardless whether your instructor would allow it or not, you may simply be covering up a bigger problem.
Also, most tape adhesive will damage your bokken's finish, which would be the primary reason I would never use it myself. Unfinished wood naturally absorbs liquids like oil and sweat, so it is important to remove any lacquer finish that may prevent your bokken from absorbing fluids; that may help your problem.