Yes, US traffic is considered to be a great traffic source for conversions and quality but you also need to consider what exactly you are trying to market and sell. You need to think about the item, whether it's a product or service, and who would be the best choices for people thatwould be potential customers.
If you are trying to sell something that isn't a big seller in the US then seeking and getting only US traffic isn't going to be the best choice.
This is all going to depend on what exactly you are marketing and who your potential buyers would be and depending on those factors, that will help dictate who you would want for your traffic source.