Even if you put the actual QNH in before TOD?
I agree with Banana Joe, it's very important. FMC is all about garbage in, garbage out. If you don't supply the QNH, it will assume it's standard, so in case of very high or low QNH, the calculcated VNAV path could easily be wrong by 1000ft on either low or high side - until you switch from STD to QNH.