I'd only mark a chick I couldn't tell from others. Now, when I first decided I wanted chickens, I just KNEW I wouldn't be able to tell 'em apart if I got all the same breed. Like, if I got six RIRs, I'd have to name 'em all Rhoda, and every morning would be like this: "Good morning, Rhoda. Hi, Rhoda. 'Mornin', Rhoda. Well, howdy there, Rhoda..." So I didn't; I got all different breeds.
Then I bought two laying Golden Lakenvelders, and worried about telling THEM apart. But I could. So I got brave and bought two Wellsummer chicks this Spring when I got more chicks. I could tell them apart, too.
However, now that the two Sebright roos are about 13 weeks old, I find I have trouble telling George from Alex. I have to see them together to notice the difference. Luckily, this is not difficult - they usually hang out together.
What I'm trying to say is that you should be able to tell 'em apart, really, right? Why mark 'em?