If you open up and run your worst case workload and then take a look and see how it's going.
To be honest unless you are running multiple VM's or using very RAM hungry apps then 8GB is usually a great amount.
I just tried that and with Photoshop, iMessages, iPhoto, pages, Powerpoint and 2 Safari tabs open along with Netflix i get too:
7.97 GB of memory used
and that for me is worst case scenario, i usually only have Pages open along with a couple of internet tabs at any one time, i usually focus on the one thing i'm doing at a time and try not to have too many things running in the background.