What options do I have to seal the top of the concrete base of a galvanized steel post? Should I seal it?
A galvanized post that is "fixed" into concrete should not be an issue with regard to rust, it would probably rust on the top of the post first. But if you are concerned then the main things to consider here are:


Can water get into the post and sit, normally there are holes drilled
for water drainage is the galvanized coating damaged in any way.   
Normally when I handle galvanized metal,  i may grind or weld near the posts. 
if this happens and you expose the raw metal, it will become a rust
spot and spread over time 
Will the post separate from the concrete.This is common issue when setting a square post into the cement, its always better to have a t-piece welded to the section of post that you would set into the cement.  
Lastly, if you are concerned about pooling water around the base of the post, then its a good idea to set a slope on the cement so that there is runoff. this can be above or below ground and will work as effectively.  There is no real need 
to seal the cement, since it is porous any way and will absorb moisture from all directions, but if you are really concerned, then you can add some additive (acrylic bonding agent) into the mix to make it more waterproof