If in any doubt and it's a 50/50 route, I'll always ask the customer fer their preferred route whilst offering my own suggestion at the beginning of the journey, it can save a lorra hassel at the end.
And if you do happen to make a bollix of yourself and go the wrong way, then just knock a few Euro off and usually they are pleased with that. Never ever ask; "How much do you usually pay ?" if you have taken a wrong route, because they will always come out with some ludicrous low quote.