Is it really hard to tap it straight ? should I just let the machine shop do it?

 

if you use studs with a hex instead of the repair type you'll need to machine .4 off the boss. i've tapped a boat load of them, use a drill press to keep the tap straight.

 

If you are simply installing collarless studs, you can use a tapping guide to start the tap squarely. They are about $15 at most speed shops, and double as a stud pulling spacer.

If you're planning to install hex collared studs, follow Ed's advice.