For shuls who always lein the aseres hadibros with the “ta’am hoelyon”, one could argue that one stands during the reading, since the sound of the leining is different from other parts of the Torah, especially at the end of each dibrah. This is admittedly not a strong argument, but maybe one can make a comparison with “oz yoshir” which is leined to a special tune and one stands. On Shavuos, there is an additional reason to stand when reading the aseres hadibros, namely, that this Festival marks the anniversary of receiving the Torah.  However, according to “Luach l’Eretz Yisroel” of Rav Tukachinski, one only uses the “ta’am hoelyon” on Shavuos, but for the parshios Yisro and Voeschanan, he writes that one reads the aseres hadibros, as for other parts of the Torah, namely with the “ta’am  hatachton”. Since the above two reasons for standing do not apply on these two Shabbosos, it could therefore be argued that one should remain seated.