I was once told that playing a guitar with greasy fingers was actually good for the strings. However, this seems a bit counter-intuitive. Is there any truth behind this claim?
Disclaimer: I'm not an expert in this, yet try to make an attempt to summarize what I know on this subject.
What does Grease consist of?
Grease consists mainly of dirt, dust and sweat.
Dirt and dust consist of human skin cells, plant pollen, human and animal hairs, textile fibers, paper fibers, minerals from outdoor soil, and many other materials which may be found in the local environment. Most of these are soft so for these it's rather a concern about how they make your string look...
Sweat consiss mostly out of water (H²O), some urea (A water-soluble organic compound, CO(NH²)², formed by the metabolism of proteins.) and a bit of salt (NaCl). Water isn't harmful at all, urea isn't either (unless you somehow manage to nitrate in which case it comes highly explosive). So our main concern should be the salt in terms of the life span of your strings, as it is a corrosive material...
Let's see how these affect different types of guitar strings.
Here, your main concern is how your strings would look, as dirt/dust/sweat can stay on your strings.
Nylon strings are corrosion-resistant and doesn't have a problem with salt.
For example, imagine a guitar that has been in your basement for a very long time, if you blow at the strings a lot of dust will come of. Unless you properly stored it inside a bag... :)
What applies for the nylon strings also applies here.
But an extra concern here is that salt is a corrosive material against metal, so technically, cleaning your guitar after practicing and performance can help to increase the lifetime of your strings.
Is it worth cleaning my guitar strings? How do I do it?
I would say, yes, because they will look less dirty and more shiny. And for metal strings you have the added benefit that the lifetime could be longer, so it's certainly something you could consider doing.
Lay the guitar on its back, resting the head stock on small box or similar, to stop the machine heads going out of tune.
Grab a sheet of towel/cloth and fold it in half length wise. Fold it length wise again so you have a long rectangular piece. Do the same thing to the other sheet. This one will be the "Oiling Rag".
Slide the dry sheet under the guitar strings just near the bridge, then fold it over so the other half covers the top of the strings.
Now, slide this piece along the length of the strings, right up to the nut. Apply some downward pressure to clean the top part, and pull up on the paper to clean the underside of the strings too. (This is just as important as the top) Even if the head gets lifted off it's mount, it still helps.
If you notice a little bit of muck and dirt on the towel, then you're doing fine. It won't always be like that, but this step helps set the strings for the oiling.
Optional, as doing it unproperly is bad for the wood: Apply some WD40 to one side of the other piece of paper towel and wrap it around the strings the same way as the first piece. Run this piece along the length of the strings using the same process as before.
When that's finished, the strings should look noticeably shiny (and be very oily) and smooth to touch.
Definitely bad. Grease is sticky, ideal for lubricating things which need high viscosity. it holds particles of grit, dust, anything, and can hold moisture in.
Guitar strings should be kept smooth, away from moisture or anything which could corrode or rust them, and you don't want grease or grit on your fretboard.
So stick to a wipe with a dry cloth, Fast Fret and your strings will last much longer.
Oil prevents metal from oxidizing. Salts are corrosive. Sweat is both!
Overall, I'm pretty sure the salt wins out. My guitar strings always do worse when I don't keep them clean, and I've always been told that they should be cleaned after playing.
I wouldn't recommend oiling your guitar strings either, though. It'll get into the wood and make it gross, as well as altering the feel and sound of the strings. It's best just to keep them clean.
The question is a bit vague as to what good and bad mean, the answers here are talking about lifespan, but it is also worth considering that as strings get dirtier it changes the sound (note - I'm an electric guitar player, things may be a little different for acoustic/classical, but I expect they'll be similar). Try comparing a brand new set of strings to an older set and you'll hear that the new clean ones have more jangly top end. Most people seem to prefer the sound of new strings, so keeping em clean would be a good thing. Me, I prefer older strings myself, so a good bit of skank makes me happy :) That said, grungier strings don't intonate so well either. Ho hum.