I tried to roughly estimate the risk.
There are 18,000 theaters that could be targets. Let's say there are 10 showings over the opening weekend that might be bombed, so if you attend, there's a 1 in 180,000 chance you'll be in the bombed theater (assuming there's only one bombing). Let's say there's a 5% chance it's a real threat (personally I'd estimate around 1%, but let's use 5% to be conservative). That gives a 1 in 3.6 million chance of being in the bombed theater.
Compare to car accidents. There are 1.27 fatalities per 100 million miles traveled in the USA. If the movie theater is five miles away, that gives you a 1 in 7.9 million chance of dying in a car accident on the way to or from the movie.
If we stop calculating there, it looks like you'd be roughly twice as likely to be in the bombed theater than to die in a car accident on the way to the theater. (Note that this is the chance of being in the bombed theater, not of dying in the bombing. Presumably, most people in the theater would survive.) To me, that seems like an acceptably negligible risk.
More detailed calculations could include the chance of dying conditional on being in the bombed theater, and could include the chance of more than one theater being bombed, and adjust for the chance of the bomber being foiled or the bomb not working, and could adjust for which theater you're seeing it in (theaters in major cities are more likely targets than small towns).