It seems there are fairly polarized opinions on this and no "right answer". If you're a good negotiator and can knock the dealer down to something near what a backyard tinter would do, then it obviously makes sense to do that as you're covered by the dealer warranty and will be able to include the tinting as part of your financing/novated lease.
If you're not such a good negotiator or your dealer plays hardball, then it's down to what you can afford and what you're comfortable with. If money is no object or is less important than your peace of mind, then get the dealer to do it. If you trust backyard tinters with a good reputation, then you can save yourself some money by avoiding the dealer, bearing in mind that you are then taking the risk of poor workmanship... so it's in your best interest to do your research properly.
I personally went with a backyard tinter because I have an objection to giving money to middle-men where those middle-men provide little or no benefit in my opinion. I do appreciate I have different opinions to some though.
I watched him do it and it was quite clever. He cut the tint to size on the outside of the window where he could, then for the bits that would be out of sight he trimmed the tint on his cutting board. He used his heat gun to preshrink parts of it so it had the right curvature (and I think this is where the skill and experience shows). He then wet the inside of the window with his hand spray, put the tint on, and slid it down the window so that there was no exposed untinted glass. He then used his little hand tool to press the tint on flat and move any bubbles to the edges and out.
I checked the trim after he'd finished as I was concerned about scratching etc, but it was in perfect condition still from what I could see. The only other concern I had was that the edges out of sight hadn't bonded to the glass properly, but have since satisfied myself that it isn't really a concern as it's unlikely, and besides, I have a lifetime warranty if it's necessary. How am I satisfied? Well, take a thin plastic film similar to tint (e.g. overhead projector sheet), clean a window then wet it. Put the film on the wet part and notice it sticks. This has something to do with water surface tension from what I read, so you can push the film around on the wet part and the edges don't come off. The tint film is a permeable membrane that bonds to the glass as the water evaporates, so the edge should be securely bonded now.
I have to agree with you Mav, it doesn't sound right that you can't take the trim off. It's probably that you can't take the trim off easily, or without having to replace clips or something, but the trim can definitely come off otherwise the dealer would not have been able to replace my broken drivers door window.
As for your comment on doing the job "properly", I guess that's a subjective call. I think my tinting has been done properly without the trim being removed, but perhaps you don't feel it's done properly without actually seeing the entire window covered in tint to the edges (even below the trim where the glass will never see the light of day anyway). As I've learned in life, there's "Best Practice" and then shades of gray through to poor practice... it's not always a case of "black and white best or worst practice.