I was just about to post something similar. It is a great away to earn money. Be sure to do so for all of your characters.
You will have to wait until the mission is over to switch to them but if you do it as soon as the mission ends they'll still make a lot of money.
For example one stock had already went up $28 when I switched to Michael. When I switched to Trevor I was immediately sent into a mission but once completed a couple days had past and the stock was now up over $120 from the original price I paid as Franklin.
So when doing those assassinations missions be sure to switch to your other characters as soon as you can and buy the stock for them as well.
As the OP mentioned it does taper off after a few days so if you switch and get dumped into a mission just check the stock page and see where it has been trading.
Also to the OP thanks for pointing out the stocks to purchase in the later missions as I will just buy those before starting them as Franklin.
Last edited by Wreckon Dracgon; 09-19-2013 at 04:18 PM.