In a newly set up aquarium, water testing is absolutely critical to avoid fish loss as ammonia and nitrites rapidly rise. In an established tank, water testing is important to ensure the continued health of your fish.
Test kits should be considered part of the operating expense associated with keeping an aquarium. If you cannot afford test kits, or feel uncomfortable testing water yourself, check with your fish shop to see what they charge for doing water tests. Some offer one free test each month, or will quote you a flat fee for monthly testing. Compare their charges against the actual cost of test kits.