As long as she's not hurting, there is no reason not to ride her. Really, her swayback isn't half a bad as some that I've seen.
Like smrobs said though, saddle fit can get expensive, and if you don't do it right, you can damage your horse's back. We have a 28 year old mare with a swayback at about the same severity as your girl's, that we still use for lessons, who- for a while, we would just tack her with two saddle pads and a normal saddle. Within a few months though she was acting up and just over-all being pissy, a sure sign that she wasn't feeling good, considering that she's a beautifully trained WP mare. Got a chiro and saddle fitter out, and it ended up costing us about $350, while we could of saved a good bit of that by just fitting her in the first place.
What works good for her though, is a Cashel Wool Swayback Saddle-Pad.
After an adjustment and the new saddlepad, as well as a fairly deep gulletted, properly fit saddle- and she's back to her happy, willing self. Have your saddle fitter or chiro recommend a saddle or a pad for you, and don't do what we did!
Just be sensitive and watchful for signs of discomfort, and condition her properly. She's got a good 5-10 years of trail riding in there!
Two pictures of our gal- Puddin'. Completely unflattering first picture, but its the only one I have of her untacked. No she's not foundering, she's getting up after a roll ;)