From a screenwriting perspective, let me give you two examples:
a) period pieces such as stories set in Ancient Rome
b) high fantasy such as LotR
In the first instance, there have been some film and television productions set in an ancient era (when English did NOT exist) and yet the characters are (of course) speaking English dialogue. So the producers chose to have those actors playing rich, educated, noblility characters speak with very eloquent strains of proper English. And then they ahd the lower class characters speak with very bad, low brow accents --such as maybe even a Cockney accent (my apologies to anyone here who is Cockney). Some spoke with American accents and Australian accents (but the general preference is to stick with just British accents).
In the second example, Peter Jackson chose the comic relief of having the Orcs arguing with each other with Cockney accents. (And admittedly, in the book, they Orcs definitely spoke in very bad "English" that seemed very Cockney-ish when you read it. In fact there was a scene in the book when Sam had the Ring on, and so he was invisible, and at the time he assumed Frodo was dead from being bitten by the giant spider Shelob. But while invisible he was evesdropping on the Orcs who took Frodo's body, and that's how Sam learned --from their conversations-- that Frodo was still alive. The Orc dialogue that Tolkien wrote out was VERY Cockney-ish. Unmistakeably so.) I think the two American actors, Elijah Wood and Sean Astin, made excellent choices with the British accents they put on. Sam's accent was NOT Cockney, but it was certainly working class. And Frodo's was a but more upper-crust (but not in a snooty way). (As an aside, I have the 10-disc CD set for the BBC radio play from the late1970's/early 1980's. That radio play starred Sir Ian Holmes as Frodo, and some lesser known British actor as Sam. Sean Astin had a friend get him a copy of those old BBC tapes, and Astin diligently listened to the accent as performed by the actor playing Sam, and he was able to copy it PERFECTLY, and THAT mastery of someone else's performance is how Sean Astin got the part.)
Now, I know you didn't ask about accents, but about contractions. And I also know you're not writing a screenplay, but a novel.
However, I am one of those readers who can "hear" the dialogue in my head as I read a novel, and if a character NEVER uses contractions, I'm apt to assign an upper-crust identity to them.
On other matters, I found the dialogue from many of the Star Wars films very stilted and fake-feeling when certain characters did NOT use contractions (or else used very contrived contractions). Such as when Luke said to Vader: "I'll not fight you, Father." THAT felt stupid and fake. Or when Padme said to Anakin (as they discussed whether they could love each other): "I'll not give in to this!"
My overall advice is: try to "hear" it in your head as you write it, and go with whatever genuinely "feels" like THAT character.