It depends on whether you're asking about operating system licence or someone building your computer.
Operating System licence:- I believe it's the same as OEM which is the cheapest full licence available, but it's not necessary, you can go retail which has more options available for transfer and product support, but is also dearer.
Someone building your computer:- if you don't know what you're doing, or you're not confident, then maybe it's the thing to do. However, you won't learn anything by getting someone else to do it for you. But, again, it's not necessary.