glow Plugs

You may find appropriate my answer here Link

I had the old 2.8D hilux and only glowed it first thing in the morning or when was about to come home from work after sitting in carpark all day. But general running around stop start here and there never glowed it up and started first go. If I was some where and the lux was sitting for say an hour or more then yes I would. I would definately warm them up first start in the morning and when leaving for home. Probably also depends on the climate, where you live etc. My glow plugs were replaced at 170k, sold it with 300k on it and to my knowledge have still not been touched since the 170. Regards Steve M

Don't know what vehicle you have but on the td42, i only glow it when the motor is cold or has been stopped for over half hour or so, with the td42 if you go past the on position (even just a little - not enough to start it) it will disable the glow plug timer.
To make the plug timer come on again you have to turn the key off then back to on again.
I came to this way of doing it by trial and error, if it ever took more than about three turns i figured i should have glowed it - and made sure i did next time.
It will start on a cold morning with no glow but that takes a few more cranks - as opposed to my old sd33 mq which had a button wired up and had to be held on for about 30 seconds, then turned over foot flat while holding the button for a further 10 seconds or so and even then would fill the area with white smoke!!
I put that down to the td42 having probably a more efficient combustion chamber design and possible higher compression.
Sorry for the drawn out answer in short no you shouldn't hurt the plugs and no it shouldn't be neccesary if the motor is hot (even with an old sd33!)

They really only need to be pre-heated when cold or if the conditions are down around 5 degrees or lower then you would probly consider pre-heating all the time. As your vehicle starts to age youl find it probly need more pre-heating to crank. Weve got a couple of old volvo trucks at work that wont start unless you pre-heat them for at least 40 seconds. Longer on cold days.