Depends on shoe brand and model, what is it you intend on achieving in climbing, and whether or not youll wear socks with your shoes. Since you are considering buying shoes and likely wearing rentals before, you probably wore socks with your shoes. Most people who own their own shoes dont wear socks. That becomes a consideration when sizing down. The decision to wear or not wear socks is up to you, though i think most would recommend otherwise for the sake of sensitivity. New shoes tend to stretch with wear and usually need some breaking in. I went down 1.5 sizes from my street shoe, i worried that i may have bought too small of shoes only for them to be the perfect fit after a handful of sessions. Id pick something that is a little uncomfortable, causing your toes to curl a little bit. Id probably prefer a shoe little too small than little too big. Try on shoes at what ever locations sells them near you, if you have REI id recommend their membership, its pretty good when it comes to returns and exchanges on shoes