When you use Megawizard there are options as to what should be registered, also the sample waveforms under Documentation will show the effect of the registers, typically each register delays the output by one cycle. If the rest of your design is pipelined, you need to see how the operation fits into the total. So if you are using pipelining to get faster clock sped in the total design you may want to register, just remember that every pipeline stage adds a clock cycle of delay so if you cannot speed up the clock enough to save a cycle, then you lose performance. For example: if you have a 5 stage pipe and you change to a 6, then if clock frequency does not increase by at least 1/5 you lose.
