Piano was my first instrument, and although I am not a professional musician, I was quite advanced on piano (when I picked up the guitar).
I picked up guitar at first, playing folk, singer-songwriter sorts of things. When I was in college, I looked for a guitar teacher, and when we ran out of folkie things to do, we migrated to classical (he was a college student majoring in classical guitar). I first had to learn to read music for the guitar. Because I already read music fluently, this came relatively easily. Also, I was well versed in music theory and had studied some improv on the piano.
I was lucky with my guitar teacher - he taught me well. I developed good technique, a good, if somewhat light, touch, and an expressive approach. I will tell you that I would never have gotten comfortable learning the various aspects of technique without the guidance I received. The problem is that you don't know what you don't know. It is not just a matter of tension in the hand, but so many other things.
We are all different, and perhaps you are comfortable with these aspects of technique. But now, after a long hiatus, I am once agains studying (guitar) with a teacher. It is relaxed. Sometimes I choose a piece; sometimes my teacher does. Sometimes I put a piece down and move to another. But my teacher's input in invaluable. He helps me work out fingerings and gets me through sticky spots on a piece. I've even written an arrangement or two, and we've discussed writing accompaniments. In addition, I've learned a music notation software, so I can record my arrangements, or edit something I'm working on.
So while I won't tell you that you should study with a teacher, I will tell you that I've found it invaluable.