1. Buy tires that have a smaller width then your wheels. The usual concensus is a 205/40 will stretch decently on anything 8" wide and up. Also the brand of tire makes a huge difference in what the stretch looks like. Do research!
2. If you have to ask how to camber your wheels you shouldn't do it. Get some more experience first with suspension bits and then go from there. Mk3's are only front camber adjustable from the factory. They can be adjust +/- 1.5 degrees (if I remember correctly) with the factory strut design. The rears are much more involved requiring camber shims or washers under the stub axle. Believe me you need to know this stuff before you dive right into it.
3. Most people on here camber their wheels and stretch their tires because they are trying to fit a wider then normal wheel under a car that was never designed for something that big. Also it looks good!
4. If the work is done right and done well by someone who knows what they are doing then sure it's completely safe.
5. You can stretch your tires as much as you would like theoretically. I have seen 195/40's on a 9.5 which is a massive stretch. Granted I wouldn't recommend doing that much stretch on a daily but a moderate to somewhat severe stretch is fine.
6. How much you should camber your wheels is all about what the fitment is like and what kind of look you want. Do some more research before you even attempt something like that!
7. Sure it's worth doing if you want to do it. However it's only for looks and nothing else. In fact, lowering a car as much as most of us on here do and stretching/cambering the wheels and tires kills the handling. The Mk3 platform is already one of the worst handling platforms out there because of the solid rear beam, but all the suspension alterations most people do makes it that much worse.
But seriously, do some more research, I don't even know why I am posting this much info. There are many threads on here about camber and stretch. Look around and knowledge up before you tackle any of these projects.