On Tue, 14 May 2013 11:01:03 -0700 (PDT), Harry K
wrote:

No, after the foundation is level there is no need for a string.
Those blocks are laid dry with no mortar to level them.

Harry K

I'd still have handy level to check the plumb.

Just in case, a string level:

http://gorambling.com/images/building_stuff/tools/string_line_level.jpg