As a general rule of thumb, do the exercises that are challenging for you. I didn't use Apostol as a primary calculus text but it has good problems. For instance there will be problems where you can see the solution almost immediately (for instance, many integrals) and it's just a matter of computation. Of course you also want to be competent enough at computation. For instance, it's easy to learn all of the derivative rules in one sitting and you'll be able to take any derivative if you apply the rules carefully and correctly. Thus, there really won't be any ingenuity involved in these exercises but the key is to compute effectively and precisely.
For more specific topics, you should use your own judgement. It can't hurt to do all the exercises but I personally think it's a waste of time because there will be trivial exercises.