Only necessary on the fronts (as Steve said) and after looking online at the miller tools compressors, they retail for $900 a piece and if someone with less than good intentions 'rented one' and never returned it, someone would be out a wad of cash. From what I understand, a more generic compressor can be had for much less, just never used one so not sure on quality. The Miller tools XF specific ones are VERY high quality.