Normally, I would advocate avoiding pre-orders, generally speaking, since unscrupulous companies tend to take advantage of people that way. For that matter, I tend to really dislike DLC as well.
Things are different in Paradox land, I think. Due to their good track record, communication with fans, and obvious skill at game design, I think that pre-ordering Stellaris will be a pretty safe bet. So will HoI4, for that matter.