Knowing the pressures will do you no good if you don't have other information. Pressures are just the beginning of the process to determine a god charge. I have some numbers that I generally look for but they are more like a range of numbers and they still mean nothing without superheat and subcool as well as a few other numbers.
So the answer to all of those questions is no. Most generally you system already has service taps installed so there is no need to add more. Also if you were to discover that your unit is low, then you still have to call a pro, and the pro is going to do the correct tests to determine that anyway. So why go to the trouble of checking your own pressures. If you dont already know this information you have no business dealing with it.
The only true knowledge is the pursuit of knowledge