Well, I plan on reading the X-books, Avengers series, Spider-Man and Fantastic Four starting before Civil War, and at least going into Secret Invasion. I just wasn't sure if I could get away with just reading those without the main event series. I'll probably read the events too, but I wasn't sure if I would be lost if I decided to just read the tie-ins from those series.
Don't bother with the Avengers tie-ins. They don't lead anywhere and are contradicted later by Bendis. The X-Men tie-in was in a mini. The main books didn't really tie-in. The only relevant FF tie-in was the first issue by Dwayne McDuffie.
For Spider-Man only the ASM stuff mattered.