Tom's point is well taken. The value of the creased covers depends on what else is readily available. If a specialist can get pristine copies of the same first flight covers easily on
ebay, for not too much money, then there is no reason to buy creased ones. However if the covers are scarce, and the collector might not come across them again anytime soon, then he will be interested if the price is right. It helps that the cachet and stamps are unaffected by the crease.
Part of the value equation is also "eye appeal." Are the faults really distracting? Or is the cover so rare or cool that you don't mind them. Could you use the cover in a exhibition? Could you show it off proudly to a collector friend?