I'm usually a 10 but go for 8 in Reiss. Some designs are a little odd though and might fit in one place but not in another. Apparently they're about to overhaul their sizes and make them a bit more alike!
If you're an 8-10 usually I bet you're an 8 there max.
Reiss is the WORST for vanity sizing. I had a pair of jeans in a size 4 one year, bought same style in different colour next year and had to donate them to my size 8 mother. You will definitely have to try it on.