Does personalizing the subject line help?

Absolutely yes. Subject line mismatch with body content is deceptive and destructive. It may boost opens in the short term but generates complaints, damages trust, and hurts deliverability long-term. It's also potentially illegal under CAN-SPAM ("deceptive subject lines").

Mismatch triggers complaints: recipients expecting one thing and finding another feel tricked. That feeling becomes a spam complaint. Even without formal complaints, they'll disengage—the next email won't get opened because you broke trust. Mailbox providers learn your emails disappoint recipients.

The subject line is a promise; the body content fulfills it. If your subject says "50% off everything," the body better deliver 50% off everything (not 50% off select items with exclusions). If you tease "Big announcement," make the announcement big. Under-promising and over-delivering builds trust; over-promising and under-delivering destroys it. Every subject line, every time.