The first definition of ‘May’ in the dictionary is ‘used to express possibility’, so isn’t it redundant to say ‘May or may not’?

(as in ‘I may or may not be there by 3:00′)

Just say ‘may not’ or ‘might not’

(as in ‘I may not be there by 3:00′)

You risk sounding hoity-toity*, otherwise**.

*And no, I haven’t checked the definition of ‘hoity-toity’
** And yes, I realize that the length of this post causes it to risk not being correctly categorized as a Stand Alone Statement