If your trekking pole is longer than needed for your shelter, you can always clove-hitch the guyline around the pole at the appropriate height. I've done this when guying out the head end of a side-opening shelter to avoid collapsing the trekking pole (lazy!).
However, if, like me (definitely a "vertically challenged" person), you need a pole length for hiking that is shorter than is required for the peak of your shelter, then you are in trouble if you have fixed length poles.
That's one reason I don't use fixed length poles. The other is that I want to be able shorten the poles and fasten them to my pack should I be scrambling or going through thick brush. Collapsible poles are also handy when stowing the poles in the car before and after the trip.
YMMV, obviously!
This discussion reminds me a little of an old "Blondie" cartoon in which Dagwood cuts a clothesline rope for Blondie. She comes in and says, "You cut the rope too short!" Dagwood responds, "That's no problem. If you cut the rope too short, you can always splice it. If you cut the rope too long, though, there's nothing you can do about it." :-)