You can change your profile visit setting at any time by clicking Account under the gear icon at the top of any Classmates.com page, then selecting Privacy. Next to Visit profiles anonymously, select the option you prefer (Normal or Quiet).
If your profile visit setting is Normal, your name will be left by default for each profile you visit. (You can remove your visit on a case-by-case basis.)
If you switch your setting to Quiet, your name will not be left automatically. You'll have the option to leave your name by clicking Tell [your friend] you visited near the top of their profile.