'shiftround' 'sr' boolean (default off)
global
{not in Vi}
Round indent to multiple of 'shiftwidth'. Applies to > and <
commands. CTRL-T and CTRL-D in Insert mode always round the indent to
a multiple of 'shiftwidth' (this is Vi compatible).
NOTE: This option is reset when 'compatible' is set.
http://vimdoc.sourceforge.net/htmldoc/options.html#'shiftround'