Dear Mellissa,
Thank you for your timely question. With spring officially here, our long-haired feline friends are once again starting their winter coat shed. We have two long-haired cats ourselves, one of which we have to shave occasionally, and the other de-mats himself. So, I guess to answer your question, it depends on the cat. If the mats are very thick and closely attached to the skin, it would probably be best to shave off the mats. Cat skin is very thin and easily cut, so take care when clipping the mats off. If he is quite difficult to handle, the best thing would be to bring him into your local veterinarian and have them sedate him and clip off his mats. The problem with mats is that grass seeds can get caught in them and start to cause wounds or abscesses under them before you really know anything is wrong.
I hope that has answered your question. Looking forward to the hot days of summer myself!