We're strong believers in the professional lawn care industry. Most professionals in the industry know how to apply the correct amount of fertilizer and in a way that doesn't cause potential down-stream problems. They know when and how to apply pesticides in a safe manner for themselves as well as for you and your family members.
That said, there are some companies that don't have the proper training and don't follow the standards set by the industry. If you're thinking about hiring a professional service, check them out. Are they members of PLCAA (Professional Lawn Care Association of America) or PLANET (Professional Landcare Network)? Reputable companies will be members of these organizations. Each state also has its own professional organizations.
Some professional companies use unscrupulous marketing tactics such as after treating your lawn during a regular scheduled visit and leave a plastic bag on your door that might have some leaves with holes on them with a note attached saying you have some sort of insect causing damage to your plant. Insects are a part of our environment and they like to eat which causes holes in leaves. Unless a plant is being completely defoliated, there's no reason to treat for that insect.
This is not to say that your landscape won't have serious problems that may be pointed out by the technician, but just be aware that this is one way of them adding to their sales. When you get one of these bags and it looks serious, call the office and have them explain exactly what the problem is, how serious the problem is, and what might happen if the problem is ignored. Reputable companies will be more than happy to explain the problem and consequences as well as what it will cost to correct.