.....not necessarily. See text clipping below from a MU2/TPE training manual . The taxi speed is more accurate now than it was in the previous versions, I won't be altering it. Its VERY common to move the levers out of BETA and above flight idle to get your speed going or you are an impatient "taxi-er". We did it all the time in cargo ops...with a fully loaded plane, we could barely move in BETA unless we got up a good head of steam first. The guy in the video below goes above FI and back into BETA @ 3:50, 4:30, 4:58...and I've seen other video demonstrating the same behavior...heck I myself with my own hands on the throttles of a real MU-2 have done it. Go ahead and work those levers!
-TK