The answer will vary depending on WHERE you are:
Desert: Lots of dry rock. Lots of ledges. Lots of snakes. Better to look before you leap over one. And, you get pretty good at judging which rocks might shift on you before you step on them. Lots of people I've hiked with have had their ankles narrowly missed by rattlers hiding under. I never plant my foot blindly.
Wet Forest: Lots of slippery logs. Lots of snakes. However, stepping on any log (and many rocks) will result in your feet squirting right off the top (slick) or punching though (rotten). When hiking in wet forests, I try to never step on a log, root, or rock unless I've tested it first. Nor do I blindly plant my foot on the back side of one. Go around, be cautious.
Temperate Forest: Lots of logs and rocks. Moderate amount of snakes. When dry, logs are usually fine to step on. When wet, they can be deadly. Know your area, judge the risk.
In general - chances are you are smart enough to judge whether a rock/log is large enough or stable enough to support you. If not, you'll learn quickly. So a blanket "never step on one" seems unfounded.