You'd be more profitable if no firm proced up overnight in my opinion.
Not so sure about that as you still have Betfair to contend with. Personally I'd rather have firms pricing up over night. If they get it wrong you can get on and if they don't you can hope that they drift next day. Otherwise there is such a thing as not having a bet if the price is not right. Seems like in the main it has been working pretty well for G with overnight prices.