Putting the steam.exe on your SSD will boot it up faster, make it more responsive and overall better.
It doesnt weight almost anything, either way, so why not.
Games that you play ALL the time, or games that have insanely long loading times should be installed on the SSD - for example BF3 (Origin, I know) is a good example of an SSD game.
Have ufn
How did you get 700GB of games any way...?