It would be a waste of money to hire a dryer vent cleaning service if your vents aren't clogged enough to cause a problem, but it could be dangerous if you put this job off for too long. You should have your dryer vents cleaned every 1-2 years, but if it takes longer than usual to dry your clothes, you might need to call in the pros sooner than you expected. If clothes smell musty after the dryer or if your laundry room gets hot while you're running the dryer, it's probably time to have the vents cleaned.
Some states require dryer vent cleaners to be licensed, but even if you're in an area where licensing isn't necessary, it's important to ensure that the cleaning service you want to work with takes vocational education seriously. Various organizations around the country offer dryer exhaust technician certification courses, and people who are doing this job should be certified for safety reasons.
Check a company's reputation on popular review sites before you work with them, and also ask for customer referrals. If a company doesn't want to give you referrals, choose a competitor.