Should a shower curb be sloped?

We have contracted to have a tiled shower built. This company uses the Schluter system and by watching a video on Schluter's website, it appears their curbs are level and not sloped. Our contractor said he's never heard that they should be sloped but I've heard some tilers say they should be. It makes sense to me that there should be a slope but I'm wondering your opinion. We'll be using a shower curtain but may decide to have a door installed later on. Thank you!