Clic Short Ugg Boots...What size should I buy?

I want to buy a pair of Clic Short Ugg boots for my Mum for her birthday. I've read on various websites that you should buy a size smaller because they stretch and sheepskin flattens after a while making them more roomy. She is a size UK6 so should I buy a size UK5? Please advise if I am right on this before I take the plunge??? Bear in mind that she will probably wear socks with them!!! Thanks