Sometime in the coming weeks I am gonna have some flowmasters welded in place of the stock mufflers. My question is should I replace my tips too or should I leave them stock?

 

You should replace all the pipes. The stock ones that mustang's come with is 2.25 inches. Go to 2.5 inch piping. And of course nice big shiny tips always look good.

 

Will it hurt anything to not change the pipes and tips because by not doing that I can save between $50-$100 (depending on which tips I wanted).

 

it won't hurt anything but most people would atleast repace form the mid pipe back with 2.5" since most of your after market x and h pipes are 2.5" and your factory tips are 2.5 a good exhaust shop could reuse them.