If an estimate is is APPROVED, it should be locked from changes.

We reuse (clone) estimates from projects ALL the time. Unfortunately what happens occasionally, is that an estimate will start to get edited before it's been cloned.

It would be ideal if an estimate would LOCK from edits, once it's been approved (and/or denied for that matter...)

You can't 'add' to a approved estimate, but you can easily edit and/or DELETE line items on that estimate.

So; to have it LOCKED if it's been approved (or denied) would be logical. That way, it forces the creator to CLONE the estimate and edit that clone, or to unapproved (or undeny) and edit that.