Alignment of baseboards is extremely important. It is a requirement that all baseboards should line up exactly whenever they are assembled.This can be done in a variety of ways. Split hinges are a popular method as are machined aligners from precision engineers. Me, I use 3mm Society products. A simple 8mm male and female plug amde of brass inserted into the end of each board. Kept in place with a dab of UHU (other glues are available) I have complete confidence that the rails will line up every time. Inexpensive for the peace of mind they give.
These are also to be found on E-Bay under various guises, although the brass one's are, in my opinion, the favoured option.