When I capo the 1st fret I often get a slight buzz on either the 4th or 5th string, or sometimes both. It has always discouraged me from using a capo. I don't know what causes it. A couple theories are that the fingerboard has a very slight radius to it, or that the transition between the thickest string down to a thinner one makes the strings between not get pressed down as much.
The discovery - I tried cutting a small piece of a business card and placed it on top of the offending string before putting on the capo. With trial and error I found that it's actually better to cover more than just the string with the buzz. On my 7-string the business card covers strings 6-4, and on my 10-string it covers 7-4. It totally solves the problem. Now the sound of the guitar is even and solid.
When I was sure it worked I used rubber cement to attach the piece of business card to my capo to make putting the capo on easy. Rubber cement can easily be rubbed off when you're done with it, so no worries there.