The Commitments has very good pricing, & I believe that The Full Monty is doing the same.
Not all shows go as low as these two but you can often save a good percentage by booking in the preview period, or at theatres like Southwark Playhouse who offer "airline" pricing; the earlier you book the cheaper it is.
But if there are no preview offers, I rarely book in advance as prices come down more often than not.
Agreed on The Full Monty and The Commitments, though they seem to be the exception nowadays.
Southwark Playhouse has unfortunately stopped doing their airline pricing, which is a real shame. I thought it was a great way to reward those theatre goers booking early.