My girl is the most awesome snake ever (not that I'm biased), but I'd never let her roam on the couch. Too many places to quickly get hidden....and I've had some experience with how quickly she can get into tight spaces as my 6 year old put her in my nightstand drawer once. In less than a minute, she was so tangled in the drawer, it took hubby and I about a half hour to free her without harm.
With that said, I don't know at what age we started, but we do put her on the floor and let her explore a bit. But I'm never out of touch reach. A few times, the kids have built little climbing areas out of toys and what have you, and she will hang out there. But for the most part, she will head to a hiding spot. So, it's not really much more than: put snake down in middle of floor, watch and follow snake slither toward couch, grab snake before she goes under couch (or other similar object).
If you think you won't be able to catch yours before it goes under something, don't do it. It's not that exciting any how.
If you are sure you can monitor and pick up when necessary, then go for it.
My 2 cents.