Basic Tuning: HELP!

Is it common for the Harmonic Balancer to "shift" so that the "0" is not really TDC?
I was asking a friend that I trust about this and was told to be sure that my TDC was truly TDC, and that it isnt too uncommon for it to be off slightly. I was told it may be worth while to find true TDC and check and remark if need be all the degrees up to maybe 40* both ways.
Is it worth the effort to make sure my "0" is truly TDC or is it typically OK to trust it?