A .01 flipflop isn't even worth noticing. Even a .1 is nothing to be worried about *edit* unless you're super sub-ohming, of course.
Chances are it's just a rounding error on your mod, you probably have a coil that's actually around .645 ohms so even the most minor changes in resistance when the coil is heating up/cooling down makes the display switch between .64 and .65.
Mod resistance readers aren't accurate enough to make that a worry. Even most dedicated ohm readers wouldn't be, I think.
And that vape shop sounds sheisty, sounds like that guy literally told you to ditch a perfectly good tank so he could try and sell you something else.