Flock1,
if your travelling long haul on a 767, pay to sit together. This is because the seat layout is 2-4-2 on TOM 767 aircraft and 2-3-2 on ex-FCA 767 aircraft. Thomson normally alocate the 2 seats to couples who have paid to sit together so you may avoid sitting next to a stranger in the 4 seats/3 seats This isnt 100% as they may be lots of couples on the flight and not enough 2 seats to go around.
Also on TOM long haul it generally makes no difference if you check in early, the seats are allocated the night before by head office and check in staff dont have much if any flexibility.