Should I stick with ubuntu precise or I can use newer versions of ubuntu? Are the charms version specific or there are no such constraints?
Thanks!
Technically yes, there are different versions of charms for different versions of Ubuntu. Charm authors usually write charms for the current LTS (currently 12.04, soon to be 14.04) .However in practice I've found that a number of charms can be deployed to multiple series's without issue. You will have to take it on a case by case basis.
http://manage.jujucharms.com/charms
Note the Series
column.