I recognize this is a question resurrected from 4 years ago. But after reading all the answers, there is something missing that I feel compelled to add for the sake of anyone who comes across this question in the future.
Adding a capo will almost always cause the tuning to go sharp to some extent. That's not a problem if you are playing solo with no other instruments that you must be in tune with. The degree of sharpness can be minimized however in a couple of ways.
The reason the guitar sounds sharp when you place a capo on, is that the pressure of the capo on the strings tends to stretch the strings by bending them over the fret. If you have a capo with adjustable tension, this effect can be minimized by using the lowest possible tension that does not mute any of the strings.
But another way to minimize the effect on tuning, is to place the capo as close to the fret as you can get it without muting the strings. The closer you put the capo to the fret, the less room the string has to be stretched out of tune. Theoretically, if you placed the capo directly on top of the fret, it would not stretch the string at all and would not affect the tuning. This is not usually practical however because it gets in the way of your fretting fingers.
Most people tend to put the capo right in between the frets or too far away from the fret like in this picture.
This common placement will sharpen your tuning the most. To minimize the effect of the capo on tuning, install it closer to the fret like this.
You may find that putting the capo that close to the fret and parallel to the fret makes it tight for your hand when fingering certain cords. So to add a little more maneuvering room for your hand, you might want to slightly tilt the capo away from the fret on the treble side of the fretboard like in this picture of James Taylor.
This is the best compromise between playing comfort and minimizing the sharp tuning effect. I put the capo just at the edge of the fret on the low E string and angle it just enough to give me the clearance I need. That usually keeps me pretty close to being in tune with the non-capo using members of the band. If I am going to play several songs in a row with the capo in the same position, I will tune with the capo on. And of course - re-tune when off again.