If you have separate rooms, (control room and studio room) then monitors are definitely the only way listen. But this takes at least two or three people. One to play the instrument, one to move the mic, and one to listen and make judgment.
If you are alone or have a single room studio, headphones may be the only choice short of recording and playback multiple times to A/B each position. If all you are listening for is proximity effect and sweetness, they work great for that. Knock yourself out.
However, understand that headphones (cans) usually give us a false sense of space, dynamics, and sonic character. While they may sound awesome, they don't give us what we need to hear when it comes to translation to other systems. Always use your monitors for critical mix decisions.
Step one should be...play the instrument in different spots around the room and listen with your ears. Find the sweetest sounding spot in the room before you even put up a mic.