I have only quite recently been diagnosed, so I am currently working this out myself.
However, it really depends what you want to know from the test results. If you are simply monitoring things to make sure that you have BG under control, then a fasting test (i.e. before breakfast) will probably suffice. This gives you a very approximate indication of how good your control was the previous day. If you consistently get a value over 7 then you really want to start wondering why.
If you are at risk of hypos (if you are a diet controlled T2, like me, then you probably won't be, but if you are on tablets or insulin then you might) then you should probably test to make sure that your BG doesn't go dangerously low (i.e. under about 3.5). I don't have any experience with this, so you should take advice from a knowledgeable person (a diabetes nurse and/or people on here). My guess would be that you should test at times when they are likely (if you haven't eaten for a while) or at the first hint of symptoms.
Lastly, you might want to test in order to work out what effect various foods, combinations of food and combinations of food and exercise will have on you. This is what I am doing at the moment, and I think it is going to be an invaluable aid to fine tuning my diet. I have barely started getting to grips with this, but the data that I have collected so far has told me a lot of useful things. If want to do this, then you are going to have to collect enough data to plot a curve, and to do that you will need to test at various intervals after eating a specific combination of foods. Typically this might mean testing every 30 minutes until the BG has peaked, and then hourly until it is close to where you started. There is a good chapter on this in
Gretchen Becker's book. If you do this sort of thing, then I am afraid you will get through a lot of strips. I am planning to shift to daily morning testing once I have worked out the details of my diet.
I would be interested to hear what people who have been doing this for longer than I have (i.e. just about everyone on here!) think about this.