Hey FadeX,
If possible, it's always best to avoid placing monitors near walls, especially corners. Walls will reflect and radiate sound, especially low frequencies, from the backs of the monitors and mangle what you hear. It can cause an increased level of low end and even some frequency cancellation (comb filtering) from the time difference between the actual sound and reflected sound.
I've also attached a picture that illustrates the comb filtering for you. Hopefully this helps you understand!
