I always use feel when tightening, I just happen to check more often everything is okay on a vibey v twin than perhaps a inline 4.
If I remember right, many spark plug manufacturers, certainly NGK, give a tightening spec on the box. It is usually something like hand tight + 1/8 turn for flat base plugs with a washer. There is a different spec given for taper seat plugs. I've never used a torque wrench on a spark plug and never had one come loose. I did have a problem years ago with a bevel 900SS when I unscrewed the horizontal plug and the thread came with it 3/8" reach plugs in an alloy head were never a good idea.
Spark plugs without tapered seats should be torqued up at different rates for new or old plugs as the washer compresses differently, in addition to that if the threads are oily or not, dirty or not will also will affect the reliability of the torque setting. If they are dry the setting can be unreliable and is risking going too far if the thread binds as dry ones in alloy castings tend to do. Also as the setting is so low (around 18-22Nm) use of an uncalibrated torque wrench that has a hi range (say 10-80Nm or 20-140Nm) is not recommended as you need to use a low range torque wrench on low range torque settings to minimize the risk of over tightening waiting for the click. For me using an uncalibrated or inappropriate size of torque wrench and waiting for the click on a plug is going to result in a thread strip. There is no substitute for feel
