When possible, study a race route. Download the USATF map (unless it has just been measured it may not be up yet).
Obviously it may be difficult to run the tangents if you are in the middle of a mass race but for those runners that have the freedom to run - you should run the tangents.
Several of the big time marathons like London and the Olympic Games have a line indicating the shortest possible path (SPR). Boston had a decorative line for the last mile + but nothing for the rest of the route. Runners were all over the place.
The London Marathon is always looking for fast times and world records. It surprised me to see the last refreshment station was on the outside of a bend on the road which forced runners to run wide to get their drink, then back to the SPR.
Every little bit helps - just ask Rod Dixon.