I grow nasturtiums every year, but I've never pinched them back. If you planted the standard 'vining' type, it will develop into lovely long vines. As long as it gets plenty of light the leaves will be close together along the vine, creating a nice 'bushy' appearance. If you planted one of the dwarf/bedding types, I just let them grow. They manage to become very lovely little bushy plants without any intervention at all by me.
One thing about nasturtiums is that they don't do well if you fertilize them. Mine, however, are in the ground, and I guess yours must be in containers, right? Container grown plants might need a light feeding (about 1/2 the strength recommended on the label of whatever product you use for fertilizer) when they reach the size yours are now. Nutrients can br washed out of the soil in a container, which rarely happens to in-ground grown plants. BTW, I find your planting schedule a bit confusing. I plant my nasturtium seeds in March. Most people plant them when it's time to plant peas, either February or March, or even much earlier, depending on the weather patterns where you live. Why are yours just now coming up? Did you just recently plant them?
Anyway, in addition to not doing well with a lot of fertilizer, they also don't need a whole lot of water. Certainly you should give them enough so they don't ever wilt, but nasturtiums don't seem to need as much as most flowering plants. I guess I would say give them plenty of sunlight and plenty of water, but be careful not to overwater.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams