When you start having some customer paying each time, others skipping a week here and there and then hopefully catching up later, and still others on monthly invoicing, some paying cash, some writing personal checks to you, and so on - pretty soon you have no real system in place, things easily get over-looked, mistakes become inevitable, and everything starts taking way longer than it needs to.
Every legitimate company has a consistent clear system in place to handle their billing on the terms they have established. When you let everyone just decide when they feel like paying you, pretty soon things turn into a big mess - most likely resulting in you getting screwed.
Even when you are small, get a clear cut system in place and stick to it. It is MUCH easier to see who owes you money and plan for when it should be coming in when you are consistent in your processes.
In fact, while you are small is the best time to be getting all of your systems perfected. You have more time and it is easier to change things that aren't optimized. Always be thinking, "if I had to do this task for 10 times as many customers would there be a better way to do it?" If the answer is yes, then start working to implement the new system while it is still easy.