It doesn't really matter as long as you look smart and professional. Better to go with more external layers than less, if you are warm you can take them off and stick them in the ball bag, whereas if you are cold you are stuck until at least half time. Whatever you do DON'T put extra layers under your kit, at least not more than you would normally wear when in the middle / on the line. Remember if one of the officials goes down injured you'll be on, and there won't be the opportunity to start stripping off leggings / under shirts, etc. So not only will you look like the Michelin man you'll be very uncomfortable by the end.