Long time Apple portable user here as well (my first Mac was a Powerbook 170) and far as I could remember keyboards do mark their screen if unprotected. I recommend Radtech's Screensavers
I had mine since I got my Powerbook G4 more then five years ago and I am still using it today. The cloth is surprisingly durable even with multiple washings and still looks good even after all these years.
I have that RADtech version, too. It is definitely more robust/stout compared to the ShaggyMac. The ShaggyMac is a lot thinner, which I prefer.
Also, the embossed "RADtech" logo was imprinting it's own smudge on my screen which personally drove me nuts. Probably because I was inserting the cloth "upside-down"; user error.
Either one is better than having nothing, though, and just a matter of preference.