TiminCincy,
Major risk is bricking the router.
If you decide to go with Tomato there are many features not available in the stock firmware. If your only concern is QOS, try the setting it up in your router with the stock firmware. If it works you have a working solution. If the other features tempt you or the QOS on your router is not working to your satisfaction, then you can consider an alternate firmware.
In addition to the QOS, I use the OpenVPN to connect my office to my home. I have an extended network topology using the same ip range over the internet. This simplifies my the setup of Voip, file transfers, network cameras, network printing, etc...
There are many features, many of which you won't be needing right away. There are a lot of resources available on the internet for working with alternate router firmwares. Best thing is to do some reading and decide for yourself.