There is no "maximize" function in Tk. You could however use your method, which I would use like this:

$main->withdraw();
$main->geometry($main->screenwidth."x".$main->screenheight."+0+0");
$main->deiconify();
$main->raise();
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First you minimize the window, then you set the new geometry, after that restore the window and then bring it to front.

--HolyGrail