Damn... 5-6 feet?!
Well, I guess the plants can't complain you don't give them much room...
As for me, I'm still trying to figure out the best spacing. Last year I did 12" due to lack of space. By summer, the plants were all invading each other's territories, all doing well. Except poor little serrano, which was the first to set fruit and was produced several at the beginning, but once the cayennes all took off, it was shaded badly and didn't do much for the rest of the season. It seemed like the serrano was more interested in flowering, while the cayennes wanted to grow first.
12" in my experience is definitely not optimal, and you could have problems similar to mine if you do it, so I wouldn't recommend it. On the other hand, all the plants that didn't get too badly shaded by each other did very well, but by harvest time it was a PITA to reach between plants and get peppers off of them... mostly because I should've put a space every two rows. I didn't expect them to grow and bush out as much as they did, considering how small they were when I bought them (it was my first year growing).
This year, I'm trying something different. In a bigger garden that gets far more sun (last year's sunflower garden) I'm planting all plants 16" apart, with 21" between rows, and in a zig-zag format as an attempt to minimize shading and make it easier to reach each plant. The other garden, last year's pepper garden, is also be occupied by a few bell pepper, jalapeno and habanero plants, but all spaced around 13"-14" apart. The downside? I can't plant any sunflowers this year, because both of my gardens are being completely occupied by peppers.
I'll see how well this layout works later this season, but either way I think it'll be a lot better than last year's... it was much better planned, and has far more sunlight to begin with, not to mention the extra room to make harvesting easier.