SENSE OF HUMOR ALERT:
No, its a bad idea to use non-canon/canon ships in your sig. If caught you can be sued and do some serious jail time.
[Background: Helicopter circles my home, police sirens and loud speaker squeal]
Knock, knock, knock:" Klingon Fanatic we know you are in there. Come out with your hands up! We know you are using a copyrighted ship in your signature. Let the Klingon Academy Suvwl' Qeh down and nobody will get hurt. Come quietly and we'll talk to the DA about giving you a life sentence vs the death penalty!"
SERIOUSLY:
As a general rule, aslong as you aren't selling the ship in mass quantities and seriously threaten the profit margin of Paramount or any gaming company or individual ship maker, I think most folks would look at these ship sigs as free advertising. As a general rule of thumb if a ship maker, such as somebody from a 3D Trek site (e.g., Sci-Fi meshes) asks you not to use their ship its a good idea to be respectful and not use the ship.
Having written this, I can't say I've ever run across a situation where somebody wanted their ships not used in a sig.
I once had Daniel Brezgonne ask me NOT to use his ORIGINAL designed Tholians in a SFC Role-playing thread I started once because I was not representing the Tholians "correctly" in terms of established SFB rules/canon/non-canon behaviors. I respectfully stopped using them as part of that [to quote Brez] "travesty". I have a great deal of respect for his knowledge of SFB Tholians and learned quite a lot since that experience. Now if I ever use Brezgonne's Tholians again in an SFC RPG thread I can misuse them in new, more agrivating ways, LOL! Just kidding Brez, your Tholians rock!
There are a number of good threads about FAIR USE of copyrighted material, Copyright Law and Copyright Infringement issues scattered throughout the GENERAL and MODELS form if you want to SEARCH for them to read more about this fascinating topic.
Happy gaming/reading.
KF