A tach takes the guesswork out of tuning. For break in, there's little benefit because it's easy to hear the transition point from slobbering rich to smooth operation. It does make it easy to get tuning right because you can find your peak rpm, richen up, then point the nose up and find it again. That will tell you how many rpm off peak you need to tune to never go lean, so setting your needle valves as the weather changes gets really simple.