I think it's really good idea. Spark plugs are one of those times when torque is pretty important. You want them tight enough that they hold pressure (and pressures in the combustion chamber are very high), and also not back out from vibration (which engines do). But you also don't want them so tight that they'll never come back out, or break on ya (looking at you Ford).

Now, if you're a mechanic, hobby engine builder etc., you've probably developed the right muscle memory, and you can get them close enough without a torque wrench. If you're like me and you can count the number of times you've done it on your hands, then it's unlikely you have the experience to nail it.

If you don't have a torque wrench on hand and you need to get the engine back together, then you can get by with dad2pwd's advice. But I would still go back and re-tighten them with the torque wrench at my earliest convenience. It's a handy tool to have anyway if you plan to fix your own car.

Don't forget to always finger tighten first so you don't cross-thread.

Torquing to specifications is always preferable, but sometimes it isn’t possible to get a torque wrench in the space available.

If you can’t get at it with a proper torque wrench, check with the spark plug manufacturer. They will often give an alternate option; something like hand tight + 3/4 turn.  

If you aren’t sure, don’t guess.  Good luck!

It's best to do so but I'll admit that frequently I just snug them up and have not run into any problems. You have to be especially careful with aluminum cylinder heads. Old-school cast iron heads are much more forgiving.

I would say it depends on how easy it is to get to the plugs. I watched my step dad change spark plugs probably a half dozen times when I was in high school and college, and he helped change them on my HHR when it hit 100k miles. He more or less eyeballed where I was when they were finger tight, and then indicated when to stop with the 3/8" ratchet. Around 1/2 to 3/4 a turn tighter. I never had an issue with that car after doing the change. As Chuck said, your biggest concerns on new cars are both accidental cross-threading and stripping out an aluminum head.

I've followed his method the three other times I've changed plugs on vehicles in 10 years, and only had an issue recently with my '79 Pontiac because I couldn't get the ratchet to move very well under the A/C compressor. The #2 and #4 plugs worked themselves slightly loose over 6 months because the available room wasn't enough to accurately gauge how far I twisted them. Not being able to see compounded the issue. Gas smell and a poorly running cold engine prompted me to check them. A couple swivel joints got the drive up high enough for me to do it my usual way and they've stayed tight.