2005 NEC 110.14 (B) "...shall be covered with insulation equivalent to that of the conductors or with an insulating device identified for the purpose."

I usually use a few layers of glass cloth tape (varnished cambric is good too) which helps prevent sharp edges from poking through the tape, next a layer or two of self fusing rubber, followed by at least one layer of vinyl electricians tape. The glass cloth or cambric will also make it easier to remove all the tape should the connection ever need to be taken apart.

Few things are harder to put up with than the annoyance of a good example.