I think if it's possible to know what the choices are before you get up to the counter, you should be ready to order (without dithering) when you arrive at the counter.
But, I've been to a lot of stores where you walk up to a counter to order that actually make it rather difficult to know what you want when you get there. Stores that display their selections often put them behind the counter (where they are hard to see, even if you have good vision) or have the line for the counter right in front of the display case.
There's a bakery/coffeeshop that my husband and I go to sometimes that is extremely popular. Their set up is that they have a long glass display case where all of their cakes and pastries and such are displayed, and there are usually 2 or 3 staff members at that display case taking orders for food and for any drinks. Because of the way the store is set up, the only place for the line to the cash register to go is right in front of that display case. It's often crowded enough that the line wraps back on itself, so you often really can't see what's available until it's time to give your order. There's also a lot of variation in what pastries and cakes they actually have available on any given day, so even regulars won't always know exactly what they want until they can see what the choices are.
In a situation like that, when that particular bakery is crowded, I think people should try to avoid dithering as much as possible, but there's going to be a certain amount of that no matter what. There's really nowhere to stand off to the side to make up your mind (and you wouldn't be able to see anything to make your mind up, anyway). People could probably let others go ahead if they were still deciding, but often the people behind them also wouldn't know what they want because there'd be no room for them to stand in a spot where they could see what is available.
But in general, I do think people should do whatever they can to make up their minds before they get in line to order, and at the very least, know what they want to order by the time they reach the counter. To go along with that, I really wish that more stores made paper menus you could read while standing in line available, or at least had lists of what was available displayed in a spot you could see before you got in line. The bakery I mentioned above would probably be a lot less chaotic if they had a list of their baked goods displayed outside the shop next to the entrance. It wouldn't necessarily allow people to make up their minds completely (since visual appeal is important, too), but it would help.