I've never used a lower vice block either. There's really nowhere that you'll be torquing down on. Now, upper vise blocks are highly suggested when tightening your barrel nut. I use this one though I'm sure you can find something cheaper.
http://www.brownells.com/gunsmith-t...cks/ar-15-upper-receiver-block-prod17094.aspx
http://www.brownells.com/gunsmith-t...cks/ar-15-upper-receiver-block-prod17094.aspx