It is worth selling online as well as onsite, but not worth doing any PP to the images before putting them online.
A few of the big event photography companies (and me) have discussed this a bit more in depth and we all seem to average about £20 per month per 1000 images online. May not sound to much, but when you have hundreds of thousands of images that soon mounts up.
Some give better returns than others - I can earn a lot from my social events online, and all these images have already sold once at the event as well. Sports events vary from sport to sport, for example I getter better returns from a road race than I do from football. Equestrian onsite out sells webs sales by a huge degree.
But none of these images have received any PP before uploading, as the key is getting them online as opposed to making them look good.
The public is generally conditioned to understanding that any image bought will be tweeked before it is printed and so will accept proof versions online. If in doubt simply say that prints bought will be optimised.
Also, it soon hones your camera technique!!