My personal opinion...no 'closed' system heating system likes air; the only way to keep air out is if it is truely closed. A bladder tank does that. Each time you open yours up, drain the tank, you introduce new air into the system. This slowly gets purged out, but in the process provides oxygen to rust any ferrous parts. A bladder tank can last 5-years or more, and are fairly inexpensive.
The way to size the thing is to determine how much water is in the system when it is full, then specify how big of a swing you have (i.e., from cold to max hot). That determines the volume you need to provide for expansion. Watts makes a decent tank and has a sizing calculator. Useful, even if you choose a different brand. http://www.watts.com/pages/support/sizing_ET.asp
The best place to install it is on the feed side near the circulator, but it will work anywhere in the system. this way, when the pump turns on, it has a little extra volume available.