Okay, putting aside the "change it the same time as your chain" dialogue...
Will factory steel sprockets actually show wear, thereby showing that they should be tended to?
I'm doing the chain, but my sprockets look pristine, with no signs of wear either in the seats nor on the sides. And remember, I asked for more than "change it when you shange the chain"... I'm hoping to save some cost.
If the teeth aren't obviously hooked (assymetrically worn sort of into shark-tooth shapes), I'll usually throw a new chain on them. I find new chains on old sprockets don't last as long as when you put them on new sprockets, but I personally mostly run 2 chains per set of sprockets without any trouble (and even factoring the shortened life of the second chain, still save money over new sprockets every time).
big
If you lay the sprocket flat on one side on paper and trace the profile of the teeth all the way around, then flip it over on the other side the profiles should match. If they do there is no significant wear, if they do not, the mis-match will indicate the amount of wear, and that should drive your decision to keep or replace.
Just in case you want to know what a worn sprocket looks like, here's some I prepared earlier:
(http://farm4.static.flickr.com/3117/2535181995_ff7524b213.jpg) (http://www.flickr.com/photos/bigiain/2535181995/)
"shagged, completely shagged, and oh-my-god-how-did-he-get-home-with-that?"
big
If the old sprocket does not have much wear you can re-use it. will it wear out faster than the rest?
Yes
once you have a worn component in the system then that part will wear the other new parts out at a faster rate than if all were new. That being said as long as you are okay with your other parts wearing out a little faster go for it.
On single sided swingarm bikes it is quite common for people to try and get two chains/front sprockets from the rear due to its cost.
The actual reply to your question is yes. The stock steel sprockets will wear and will show their wear just as an aluminum sprocket would. The main difference will be that the steel sprockets will typically show a bit less wear than the lighter aluminum ones, all things being equal such as wheel alignment, conditions (wet, dry, dusty, muddy), riding use or abuse and the like. All the same stuff applies such as hooked teeth, worn sides, and sharp pointy teeth all indicating that the sprocket(s) are toast. As mentioned above, if the sprockets look good then run 'em. If they show significant wear then you know the answer.