well, it is always "best" if you don't ever have to cut into the hairline, however, I've done it many times out of necessity. perhaps try putting your hair piece upside down on a styro head, place the template over it and move it until it is best situated so that your template isn't lining up in a bad spot on the crown.
you may need to move it more to one side , or back a bit further.
DebbieS.