A medium won't fit every horse. Even with a changeable gullet, there's only so much you can do, but it should fit a good number of horses. Unless you habitually ride draft-crosses, friesians, large warmbloods or the occasional really broad quarter horse, you should be alright.
If you are this unsure I would definitely get a qualified saddle fitter out! Too many horses have problems due to owners innocently using incorrect saddles and coursing bruising and all sorts of problems! Its a very dangerous business...
I wouldnt risk it personally and it hardly costs anything for peice of mind that it brings you!
Agreed, a saddle fitter is a good investment. There's also a good saddle fitting book out there (I forget the name) that is chocked full of useful information, I know you can find it on Amazon or maybe at your local tack shop. Don't be surprised if you end up going through 3 or 4 different saddles to find the one best for you and your horses...finding the right saddle can be quite a journey sometimes, much like buying a new car:P