I just flew to Africa from Denver via Atlanta on Delta (13th Jan 2011) with an SKB double bow case and had no problems, case was checked in as normal and away I went, no hint of extra charges due to size. I also checked in a large snow board bag at the same time which is certainly oversize and again no issue with this either, although both oversize cases were well under the 50 lbs per item limit.
I think weight is more of an issue than size, as long as your cases are easy to handle and transport you should be good to go.