No. All you absolutely need is one minute to answer the mandatory call GV will make to verify your number during the signup process.
If you can live with the remaining nine minutes (from the ten provided) to receive or make any important calls in the 24 - 48 hour window in which your number is on T-Mob you'll be ok. Almost no-one on the big 'Porting to GV - my Experience' thread adds to the initial ten.
And, if you do need more than ten, you can always: add when needed, I think minutes add to your number immediately upon purchase, or just watch your T-Mon phone, if you recognize the number don't answer but instead call the number on another line "I saw your number come up but I couldn't get it..."