I usually stand at the corner near the Uno's that is near Fenway Park.(Same side of the street as Fenway, Unos is kind of in the middle of two streets) I always have a sign and I have seen my friend run 3 times with no issues.
Things will be different with security for runners for sure (i.e. no bags) but I think they're still working through the crowd control issues in the city, last I heard.
Fenway is about mile 25. Here's a map. Security will be higher the closer to the finish line you get. I live right near there so it should be interesting. www.boston.com/sports/marathon/course/map/