I am working on 2 projects right now. The one that is all garter stitch, I've been slipping the first stitch knit wise. I like how it looks on the ends... and it's how I thought I was supposed to do it. However, when I started doing that on my second project which has a couple stitches of garter on the edges but the main part is a mix of knits & purls, I had to start slipping the first stitch purl wise. Slipping it knit wise was keeping the garter edging too tight and the edges were tight and the middle sagged. I don't know if I'm making sense, lol, but I think if I had anything other than garter I'll slip the stitch purl wise to prevent that edge from being tighter than the body of my work.
Ultimately I think it's whatever looks good to you tho.