I've been reading about the net regards the shares thing and it would apear that rates are not the same across the board for every player.
The LCN ones change depending on how YOU play YOUR game.
BAWSAQ fluctuate by the whole GTA Social Community.
However in saying that I thought I'd put it to the test. Invested some money in car insurance. Then drove around crashing into cars, blowing them up, stealing cars etc and in theroy the car insurance share price will increase due to the increase in car insurance claims etc. turns out I failed misserably, got killed by the cops and lost more than I invested on hospital fees lol.
Also the time it takes can vary for player to player. I'm not sure how long it will take for all my antics to take an effect on the shareprice. I might turn in on later, a few "in game games" later it might go up as the people have put thier claims through etc. Depends on quite how realistic it is.
From what I've read you can do things like invest in the Taco place, then go around robbing/killling/blowing up the chicken shops. The chicken shop share price will drop resulting in the Taco place share price increasing.
In saying all that, I'm gonna stop pissing about with the shares now and just finish the game. Will worry about it all later when I have more money to play with.