Generally, after the tank is completely cycled ammonia and nitrite are tested for mainly when there is a problem with the tank and after adding new stock to check for a mini cycle. They are actually IMO, the first thing that needs to be tested for if you suspect a problem in your tank. NitrATES on the other hand should be tested on a regular basis until you are familiar with how they rise with your set up. Depending on your stock level, a regular basis could be weekly, biweekly, or monthly. I wouldn't go more than a month without checking the trates until you are familiar with your tank. Some ppl go off of the amount of nitrates to determine when and how much water to change. There are a lot of things that you cannot test for in your water that make up the total dissolved solids, so trates can be used as a guideline. Generally I try to keep my trates under 40. Some of the more sensitive fish do not appreciate trates as high as 40. With the exception of the discus, your stock list should do fine with trates @ or < 40. Remember as well, your tap water may have some trates in it as well, so you may want to get a base line reading on that.
A side note, before adding discus you will want to make sure your tank is fully mature (the same with teh german blue rams), and even then.. you may want to rethink the discus all together because of there high demands. They may not work well for you in a community set up. Of course, I have no direct experience with discus.