No, it's cheeky, so YABU. You don't ask. Why do you think your time is more important???
However. having said that; quite often if I HAVE got a fairly full trolley and someone has only got 2-3 items I will let them go first. But NOT if there are 10 items or less checkouts. I will only do it in Lidl and Aldi where all the checkouts are the same.
I would never ever ask. If someone got to the till first, then that's my tough shit.
One time, there was a woman with a trolley with about 35 items in it in front of me, (I had 4 items.) The guy behind me had 3. The woman behind HIM had 4 items too, and asked the woman with the trolley if she could hop in front of her. The woman said 'errrm ok I guess so.....' with a bit of an awkward look. 
I said 'err NO actually!' with a
kind of look.
She said 'well SHE said I can go before her so fuck you.' 
I said 'NO you don't! You going in front of her results in your pushing in front of me and this gentleman behind me.!' The man behind popped up and said ' yeah I am not happy with it either, wait your turn.'
The woman SHOVED me out of the way, and said 'move! this lady says I can go in front!'
The checkout operator then said (to her) 'I am sorry madam, but you can't push in. Can you stand behind this lady (me) and the gentleman behind her. She still insisted that she was 'allowed,' as the woman with the trolley (in front of me) said she could go in front of her.
She could not compute how and why she would also be pushing in front of two others... (as well as the woman with the trolley who she had asked.) When the checkout operator stood her ground and wouldn't serve her, she called the checkout operator a stupid cunt and stormed off out of the shop.
Silly bint.