Should I avoid using first name only in the "To" field?
There are mixed thoughts on personalizing the subject line. Some people feel it looks too much like bulk mail if the recipient sees their name in the subject, while others believe that personalization can increase your open rates because the recipient will see the email is addressed directly to them.
It is probably best to experiment with your email campaigns. Try one campaign with personalization (mail merge), and the other without it, and send each message on a similar day and time. See how the results compare to determine what works best for you.