See I messed up as far as Lester's assassination's go. I have major OCD when I'm trying to 100% a game, so when I see side missions pop up it's like I have to do them now so they aren't on my map anymore lol. I wish I had the discipline to wait until after the main story, but it's alright, I still made out pretty decent by investing what I had at the time.
I still have about 15 or so story missions left to go which I want to finish by the end of the weekend. I got side tracked doing all of the extra stuff - last night I finished the Leonora Johnson murder mystery side mission by collecting all 50 scraps of paper scattered around San Andreas and confronting the killer.
After that, I decided to take a plane around the entire map a few times so that I could fill up rest of it 100%. Took a while but it was worth it, no more annoying grey areas left on my map