I am a half size and I order up as far as the arch if you have a high arch I wouldn't recommend them I have some of with the high arch say their ok if a normal arch or flat footed as I am I highly recommend them I wear nothing but crocs and just remember this is only my opinion
I don't wear shoes that have a lot of arch support as they bother me, but that is just my feet. I find crocs to be one of the most comfortable shoes to wear. They used to make them in combo sizes like 8-9 which fit well, but I now find 9 too big. I cannot do 8 unless I don't use backstrap.
I don't have much of an arch support and these Crocs are great the arch is perfect for me just the right amount. Before I bought my very first pair of Classic Crocs I got as much info about them as I could and what was always repeated was they never make them in half sizes and if you are a half size to to the next size like 8 and a half go to 9 (thats my size) and they fit perfectly.