I've been a shopper for 20 years. How I choose shops changes depending on my needs.
Currently I mostly audit gas stations. I like doing these shops and after completing a couple thousand I would say that I'm pretty good as an auditor and fast. When there are not enough audits, this month for example, I will fill in with cell phone audits, bonused grocery stores, random cell phone shops, the odd phone call, and hell I even did the shop where the purpose is to look for mis-sized items.
I have a formula that I use loosely to choose shops. The pay should be equivalent or more than the number of minutes it takes me to conduct the shop. This formula is only for shops within a 10 mile radius. For example, if a cell phone shop pays $17, and it is within 10 miles, I may accept that shop because I am fairly certain that I will be in the shop for less than 17 minutes. However if the shop pays $8 I most likely would not take that job because it's doubtful that I would be out of that shop within 8 minutes. I generally accept shops that I know will be pretty fast. Some shops require 25 minutes inside the store and I'm not keen to complete those shops unless they pay $25.
I also expect to get paid $1/mile outside of 10 miles. So if the shop is 15 miles away I'm only taking the shop if I get a $5 bonus. If the shop is 100 miles away I ask for $100 bonus.
I also do route shopping almost exclusively. If I'm driving 15 miles away then I'm probably doing at least 4 shops minimally. My time is very valuable and I'm not going anywhere for under $50. Therefore I do routes. I would say that a large percentage of the shops I conduct are bonused.
I won't do shops for several companies for various reasons. One company pays about 5 months after the shop, no thanks!! Some companies have terrible guidelines that don't explain the requirement well, some companies give too little info when applying for the shop, some schedulers are rude, some companies expect a purchase but don't reimburse, some companies are non-responsive with questions, some companies expect long written reports...I think I could go on with the reasons why I won't shop for some companies. If a shop makes me uncomfortable then I won't do it. For example, I hate shops that scream "mystery shopper" so I avoid them (probably another reason why I prefer audits).
The last time I checked I was registered with over 300 companies. This allows me to be choosy about who I work with. For anyone interested in actually making money in this field, I recommend signing up with a lot of companies.
I don't know if any of this info is actually helpful to anyone, but I would say that if you do this long enough you will probably come up with your own rules/guidelines to follow.