Every unit in Lasalle uses square or rectangular bases, even the irregular ones. Irregular units must still measure the base width of a wheel, measure facing and maintain proper formations i.e. waves, abreast, etc. No unit of any type uses a round base, so I would advise against basing them as such. The easiest way to show a unit as irregular vs. regular is to either vary they number of models on the base or to stagger the models on the base. I.E. 3 models of horse per base for regular units and 2 for Irregular, or in the case of infantry 8 models per base in 2 lines of 4 for regular and 4 units per base in a checkerboard formation for irregular, etc. All depends on your scale and base size, just pick something that you like visually and works with your number of bases. But definitely no round bases.