Not sure if fishing works the same way (due to the different messages), but in other trade skills there are items to make which have a skill > 250, so 250 +6 makes you succeed more than 244 +6. You can tell this since you don't get trivial messages when making them @250+items. In fact, IIRC, you need 250 mining plus the bonuses from some items to even have a chance at mining Deepmetal (there was a post about this at one point).
Fishing maps, the message is always "you caught something", so there's no way to tell if a map was caught "trivially" or not.
Being that close, I wouldn't take any chances and would go ahead and max out the skill before focusing on maps. (Indeed, I did exactly that... twice.)