When you are firing one handed do you keep the pistol at a 90º angle or do you tilt it in slightly. I have always used the tilted method, as it helps with control, but I rarely see anyone else doing this.

Just curious.

For me it differs. I shoot better if i slightly tilt my Sig. But for my Beretta i do better sraight up and down. I think you should just expirment and see what works best for you. Of course, the "gangster" style is just stupid and they can't hit shit that way.

Tilted inboard slightly.

Quoted: Please explain the advantage of the "tilted in slightly" method.  I can see why it would be more comfortable for long periods, but is there something else I am missing?

Theres no particular advantage to it.

holding my Sig slightly tilted always feels like I can hold it a little more solidly still on the horizontal plane...

that could be a lack of arm/shoulder strength though....

that said I rarely practice shooting one handed...

This is a good way to get the idea. Get in a normal shooting/fighting stance with your normal two handed grip. Now, drop your weak side hand. I put my non shooting hand over my chest, with my hand in a fist.
Make sure to include all the muscles in your upper body to support your shooting hand. This will add a lot of stability and alow for faster follow up shots. This is known as muscle recruitment.
I don't like the tilting method, even though a lot of well known shooters use it. At a distance the tilt will not allow you to adjust for bullet drop for one thing. There is the argument that the tilted method is more "natural", but holding a gun in your hand is not a "natural" act in it'self. This may seem minor, but tilting your weapon inboard has the the ejection port facing the wrong way. The weapon is designed to be shot straight up and down. With the ejection port in the inward tilting position the ejector is working against gravity(moreso), and the following round is being fed at an angle. This could potentially cause a malfunction. I personally like to shoot in a way that reduces the possibility of these types of errors as much as I can.
I am no expert, and this is just my opinion, so take it for what it's worth. I'm just some guy on the internet. This is the method I was taught in the classes I have taken, and it works well for me. YMMV. I don't pretend to be the final authority on anything.

Turn around, and point at the first picture/poster/clock you see on the wall.  then look how your hand is angled.  Mine's slightly angled. So that's the way I shoot.