I believe the default position for the performance of all Mitzvot D'orayta or D'rabbanim is to stand, unless there is a reason to sit. We count the Omer standing, do Hadlakat Nerot standing, put on Tefillin (Ashkenazim) standing, etc. We bentch sitting because that is how we ate and because sitting allows greater Kavanah for somewhat long prayers. We stand for the Amidah, however, because Chazal required that we view ourselves to be directly in G-d's presence at that time.<br>
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Kol Tov,<br>
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Stuart Grant<br>
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