A real mould (as opposed to a digital scan) with the material having a viscous consistency will push the teeth, therefore giving you a distorted impression. In answer to your question though, there is no guarantee, but the teeth will move less because during the day they will move into the equilibrium zone as dictated by your perioral muscles and tongue. Teeth actually move all the time, but you don't notice it, some people are also hypersensitive to changes. Another option is to adjust the bite as an interference during chewing can also be responsible. Currently I scan the patient towards the end of the day (after having not worn the previous retainer all day) and so far they have all come back fine, assuming I adjusted the bite as mentioned above.