Some will tell you it's smartest to claim your machine (or machines) with your laundry basket of dirties. Smart, sure, we won't argue that. Rude? That it is — but not all the time. When it comes to reserving machines the trick is to be reasonable. Spreading your dirty laundry across the lids of a number of machines you intend to monopolize for the entire afternoon between errands is considered inconsiderate by, well, pretty much everyone, as is trying to reserve an open dryer for your wet clothes that still have 20 minutes to go in the wash. No one likes a line jumper. Setting your laundry basket on a machine while you get your quarters or card for payment may be acceptable in some shared laundry spaces, while in others it's considered discourteous.
Consider, too, the size of the laundromat, the time of day and the day of the week. If it's a small facility with just a few machines, such as the ones apartment buildings sometimes offer, it's definitely not cool to use all the machines to accelerate your laundry chores. And of course, nights and weekends are going to be the busiest in any laundromat, so be courteous to all the other people who also need to wash their clothes, and don't hog the machines.