I've seen a number of pics with 315's in the rear without flares and I say get the flares... Then I saw one in person at the dealer (in the showroom) without flares and it didn't look so bad in real life.

Best thing to do is get your new wheels and tires first, see how you like it and if the look bugs you or not. If not then great, leave it as-is and you've saved $300. If it doesn't sit right with you then order the flares, you'll make it through a week without them if you need to