What in perhaps our side of the pond is called Passepartout (mat)?
Although I have framed for local exhibition and gifts, I have never deviated from white to off-white mats. Got to confess that for color (hybrid) prints I usually make do with mass market (Ikea type) frames that come with a mat that tends to be very white; The off-white is whichever is being used at the time and I purchase off someone else in the photo club. The thickness has been between 2-4mm.
Last time I did framing it was 30x40cm (lith) prints and it went nicely with a custom cut off white mat. Tried in one of the mass market frames and the mat was too white.
I have considered to use black mat, specially for Color or low key B&W.
Purists insisting borders detract from the image. Purists insisting that cropping is evil and printed rebates show fidelity to the negative.
Lately I have warmed up to include the whole rebate (6x9 negative 120) in 18x24cm RC workprints and I realised that I can do that as I am using a LF glass carrier. It actually fits very very well to the paper.

Even done it for some lith prints.
And I personally tend to maximise the area printed, with a border that is at most 3-5cm. Film and paper are not cheap nowadays, I paid for that real estate!
At least in some circles it's all the rage to include the rebates of film scans!