DD is 2.7 and has been trained for a few months, but regressed recently and still needs lots of support/reminding/bribes. Yesterday (only with DH) she agitated to wee standing up, and we persuaded her against. Just now, we were both nearby and she was arguing about it again, with us gently persuading her to sit, and she managed it perfectly standing up (into the big toilet) 
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I would like to discourage this but not make a big fuss and interefere with the training. Anyone had this? Suggestions welcome!