you can go as dark as you want on the rear windows, your not legally allowed anything on the front driver and passenger windows as the glass is factory tinted as standard, i tinted my own windows last week leaving the fronts as standard, and got pulled by a traffic car the same night, he was adamant the front windows were tinted as they looked dark because the rest of the car was dark, it wasnt until he ran a key down the inside of the window that he realised they werent. However alot of people do have the fronts tinted, it all depends on what the police are like in your area, but my thoughts are that i wouldnt want to pay to have my windows tinted only to get pulled and have to peel them off at the roadside and possibly get a fine.
my advice is to just get the backs done normally a medium tint will give you a nice darkness without making you look like some sort of drug dealer or chav.
tint guys will tell you not to use your windows for 24-48 hours either to let the glue cure and if you dont need to try not to drive at night on the day you get them done as the solution they use will make headlights and streetlights all hazy.
have you got any quotes for the tints yet?