I unchecked the "check for updates" box for a reason. It's just too much overhead and interrupts my productivity to have to stop and deal with an update before going and debugging the thing I'm working on. Even if the version is old, I'd rather just keep working than be interrupted.
Historically, ignoring of the state of that checkbox only occurred when a build was six or more months out-of-date, but perhaps Telerik has changed this. The general concern is two-fold: 1. Outdated builds may be vulnerable to security issues, and 2. The primary value to the maker (previously me, now Telerik) of releasing Fiddler is to get feedback and bug reports on it, which are typically valuable when users are using a reasonably recent build.