It depends on which order the flights post in. Generally they post in the order flown, and thus you get the bonuses for later flights. If they flights are similar length, then posting out of order isn't even a big deal.

The way that delays in posting impact you are for getting your new benefits for the immediately following flights (baggage allowance, upgrade priority, etc). Bonus miles are usually fine. The exception is if a short international partner flight pushes you over followed by a long UA metal flight (imagine a short LH hop that should push you over followed by a UA TATL). In that case the short flight would likely post later, meaning you would get less bonus miles for the longer flight.