If you are using sorting and paging in a SPGridView needing to persist the ViewState can be necessary on postback.  I created a custom SPMenuField where cliient side scripts would change an SPListItem.  The menu column needed to update based on the changes to stay accurate.  If a custom sort had been performed once posted back the default sort would apply.  On the onclientscriptclick method after your first javascript method include:

"javascript:__doPostBack('" + SPGridView.ClientID + "', '');";

Utilizing this method performs a postback and tells the system the SPGridView that sent the postback and keeping the ViewState intact and updating the custom menus with required changes.

Get 12 Free Hours of Training when you Purchase Microsoft Visual Studio 2008