while doing some pissed up bike mechanics the other nite, I took apart my rear wheel to admire its construction. I put it all back together correctly minus the lock ring. I didn't realise I had forgotten it until a couple of days later, when I found it on the floor.
I haven't noticed any difference without it. Do i really need it? The only time i can think it would be nescessary is going down a really long steep hill or doing skids, neither of which I do.
The sprocket will stay on unless there is a massive backwards force on it* - as you say, during skidding or lots of pedal braking.
You probably don't need one but then it's probably best to have one on, just to be on the safe side.
*mine got stuck on once, despite me greasing the threads when I assembled the bike. even a 4ft metal pole attached to the chainwhip as a lever couldn't get it off at first - it broke the chainwhip!
Not much point riding fixed if you are going to be so scared your sprocket is gonna unscrew (without lockring), that you cant take advantage of the braking capabilites a fixed wheel may offer.
"The sprocket will stay on unless there is a massive backwards force on it"
my sprocket came undone on Newport track, when I kicked back slightly in the bunch, not exactly a massive backwards force, the sprocket had had about 5 years of hard racing to tighten it up too....luckily I did have a lock ring on, although it had apparently loosened as "I freewhweeled" a coupla yards then the lockring caught and I could carry on......sure glad it was there.