An introduction to computer audio
Give them some air, against the wall or in a corner is not a good idea.
I use solo piano recordings to position them. If the arms of the pianist are too short, they are too wide apart.
A more useful advice can be found here: Audio physics.
Genelec offers a nice explanation of the bass response in relation to the distance between the speaker and the rear wall. When positioned wrong the bass radiating of the wall cancels out the bass radiating directly from the speaker.
A more advanced method is to measure the system response using a calibrated microphone.
Bass response of 2 different positions - Nyal Mellor
You can put a loudspeaker on the floor, use spikes, a viscoelastic material or stands.
Some thoughts on this subject by Gary L Koh.