the law here is simply if the employee is under your control then he is on the clock. Can they stop at any store they want to after that last stop on the way back to the shop? Can they go to the bar after the last stop on their way back to shop? Can they go to the strip club on after that last stop on the way back to the shop? Probably not, because they are in your truck loaded with equipment that you want back to the shop, thus they are still under your control. Basically you are telling them they should be back to the shop on xx minutes and are not free to stop where they want on the way back.
Where this can become confusing is in the example godzilla just posted, inwhich you give your guy a ride home. In the event that the employee is dropped off immediately after a job site then he is clocked out after that job is done. If you drop him off after dumping debris, then he is clocked out after that is done. If you go back to the shop to unload or load up for tomorrow, then he is paid up until that time. If you have to make other stops on the way to his house where he does not need to perform any duties, for example stop at a parts store, then he is paid for the extended time. For example he is usually dropped off 25 minutes after completeling work, but today you stoped at valvaline for an oil change and he got dropped off 45 minutes late. Then he is pd for those 45 minutes.
Even having a guy drive his own vehicle from site to site, legally, more than likely he should be on the clock. When he leaves site A, do you expect him to meet you right at site B, or do you allow him to take his sweet? Then he is on the clock. Do you have your guys ever fallow you in their own vehicle? then not only are they on the clock but if they get into an accident, your insurance is responsible for repairing their vehicle and injuries.
This is MA law, obviously every state is going to differ. Being that the OP is in NY, I know many of your employment laws are very similar, many are stricter their as well.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!