If you want to tackle some of your own maintenance, a quality torque wrench has become a must-have item for your toolbox - it's just as critical as your hex keys or a tube of grease.
It didn't used to be this way. When I started working at a local bike shop in the mid-1990's, it was unusual to see a torque wrench. That sounds crazy now, but at the time I think it was pretty common. So what changed?
Lighter, carbon fiber components
Mid to high-end bicycle frames, forks, and components are increasingly made from carbon fiber. Carbon fiber can be incredibly strong when you try to twist or bend it - as a bike frame does under pedaling load, but not when it's crushed.
Carbon steerer tubes, handlebars, and seatposts don't take kindly to being crushed under the clamping force of an over-torqued bolt. Manufacturers of carbon components have therefore published strict guidance about how much clamping force can safely be used.
It used to be fairly common for mechanics doing a pre-race checkover to put a wrench on every bolt and turn it an 1/8th or 1/16th of a turn. This isn't a good technique. A torque wrench allows you to use the correct torque, once, and then leave it there.
More precise measurements are needed
Even when parts are made from aluminum or steel, torque matters more than ever. For example, we used to install square-taper crankarm fixing bolts "really tight" - not exactly precise! Consider a modern Shimano Hollowtech II crankset by comparison - it's important to get the pinch bolts just right. Too tight and you strip the bolt head, too loose and the crankarm will rock loose.
Torque specs printed on frames and parts
Speaking of Shimano, they have conveniently stickered most of those cranksets with the torque rating where you can't miss it. Thomson has laser-etched the torque spec right into their stems. You'll also find an increasing number of brands printing torque ratings on their frames, permanently, where they can't be lost or misplaced.
Don't despair if the torque spec isn't printed on the part you're adjusting. Park Tool has an excellent guide with generic torque values for many common bike parts, if your manufacturer hasn't published something specific.
Availability of more reasonably priced tools
Quality torque wrenches are now available at prices suitable for home mechanics. Two of my favorites are the IceToolz Ocarina Torque Wrench, a wallet-friendly portable model great for occasional stem and seatpost adjustments, and the Feedback Range, for serious mechanics who need torque measured daily.
Take care of your tools
Torque wrenches, taken care of, can last you a lifetime. Remember they are precision instruments, so avoid dropping them. Keep your torque wrench in its carrying case or bag to keep them calibrated.