I recently spoke to my thesis mentor who is the director of clinical training at my university about this exact topic.
She said make you actually read the professors website before you email them. One of the most annoying things was getting emails asking if she was accepting students when she had already discussed this on her website.
She said some professors love getting these emails and will respond enthusiastically. Others are kind of apathetic. She did mention that some professors don't really like getting these emails but that it won't negatively affect you in the application process. A brief email in early fall isn't going to follow you during apps, most professors will just forget about them. All in all, she said go for it. It can't hurt.
Say that you're interested in their work and applying for the next app cycle. Read some of their publications before hand and come up with one or two questions. Keep it brief and simple. You don't have to send your CV unless they ask
