Usually you add the mop about 1/2 way through the cook. In the archives and in the Lets Cook section Chris has some very good tips on when to add mop, etc. The "not lifting the lid" is really a warning to not do it every 20 minutes or so.
By brining, rubbing the meat the day before and letting it marinate overnight can alleviate the need for moping as much.
Also digital thermometers strategically located can provide a very good basis on determing thermal trending for your smoker and whats going on in your smoker. When logged in your log book (you do have a log book to record what you are doing for future reference right?) you can get a better idea for the future of what to expect.
As mentioned previously moping about estimated 1/2 way through and then at 1/2 way intervals for the rest of the cook after that will be fine. Of course a lot depends on what you are moping with (sugar content, etc), how hot things are running, the size and mass of the product, etc...
Take a gander at the Tips and Lets Cook sections on this website that Mr. Allingham is so kind to provide for our barbecue smoking pleasure...he has a plethora of info out there.
Regards,
PrestonD
