Guess I'd always rather tend to overbuild a little, especially when doing things myself since the labor isn't much of an issue and the materials cost isn't usually too bad. Just the weight of the joists over time over a ten foot span will cause some bellying in the center if using a 2x 8, especially with snow load. That seemed to be a pretty shallow pitch so it isn't going to be shedding snow too easily - you get one good heavy storm with that wet snow and you'll be having quite a bit of weight up there. The verticals are also a good idea for wind problems. You probably toe nailed the rafters into the joists and that's the only thing holding the roof down except for gravity. You get a really good windstorm and you can get a surprising amount of upforce, espceially if you have a 12 inch over hang. I just used some of the scrap lumber I had left so all it cost was the time to cut and nail. If you used 4x4 posts use 2x4 pieces, they will fit snugly between the two joists. (I didn't put in the second joist on my lean to until I was up there running the purlins for the metal roof, then I realized how springy it was and decided to go with the second joist and I was only spanning about 10 feet, not 16. Good luck with the rain!!!
