Depends on what you're doing. What you don't want to do is push bacteria that may be on the outside of the meat, down into the inside where it could have a chance to grow before the surrounding tissue hits 140°. Somebody here figured out that 4 hours was the magic number for bacteria to become a problem.
As a general rule of thumb, if you're smoking at a lower temperature, say 200° to 230°, you want to wait until the meat has been in the smoker an hour before you insert the temp probe. Or, alternately, before you smoke you can cauterize a small spot on the outside with a gas torch, and insert the probe in that spot.
As I said, the rule wasn't applicable in the meat loaf situation.