Should I mill my edelbrock performer rpm intake?

As usual Rusty and YR are on point, regarding bolt/port alignment. One more thing to consider is how the faces mate with the heads, angle-wise. AJ alluded to it earlier--sometimes (pretty much any edelbrock intake I've used--my luck) there is more gap at the bottom of the rails than the top, or vice-versa and they can be off front-to-back as well. If this isn't right, or real close, you'll have sealing problems.
Go look on Hughes' site, they used to have a tutorial on checking this using those wax-wire strips. I've always just used pieces of solder, it crushes down good and the caliper doesn't wreck your measurement so easy. It's a simple process and worth the time to get a good seal. Gaskets will only get you so far if the gaps are off..