The simple answer is....maybe. If a case is too long, it will begin to "crimp" the bullet which will affect accuracy. Most custom rifles
have enough freebore that we don't have to worry about that. When fireforming a std. round into an improved or ackley version, the case usually ends up shorter anyway.
Like you, I fireform 25-06 brass for my 6.5-06 AI. I have always trimmed my brass to the same length just for the sake of consistency. I ran into a gentleman on this site who necks down 270 Win brass and then fireforms. Dives him a little longer neck for better bullet seating. I plan on trying that next.