It won't damage the PSU if done correctly. You would need to read the warranty to verify if it would be voided. Another option is to get a PSU power tester like the Rosewill RTK PST: http://www.newegg.com/Product/Product.aspx?Item=N82E16899261023&Tpk=rtk%20pst
Edit: What's nice about the tester is that you can connect a variety of cables and you get voltage readouts for all the voltage rails. I have one and it's helped me ID some bad PSUs. The only bad thing about either method is that you don't get a test under load.