Taking a compression test is always a good thing, when your buying a used car. When the car is cold does it start right away? Does it have good power? Is it smoking out of the tail pipe? Does the engine and run smooth quite? What does the oil look like. Take off the oil filler cap, and look inside, do you see water? Does the oil look like Coco. If any one of these thing are present.Don't buy.
You should make a general inspection, a compression test can help you to determine the condition of the engine but even having a good condition it doesn't mean that it will not fail, many things can fail from the electrical system to the engine or transmission, but if the car does not make good in the compression test you can have a strong base point to get a better price.
By the other side if you are buying it in a very good cheap price, you shouldn't worry much.