You don't have to upjet your carbs, but you could if you wanted to. Because straight pipes can handle a larger volume of exhaust, you can tweak your carbs to allow more air/fuel into your engine to produce more exhaust. It's part of the fun, but entirely optional.
It's a different story with electronic fuel injection though. EFI engines constantly monitor exhaust gases and readjust the air/fuel mixture and ignition timing to allow for optimal efficiency. An uprated exhaust would cause the EFI to freak out, especially if the catalytic converter was upgraded or removed completely.
None of that applies to carbs however.