Infective endocarditis generally develops when there is a pre-existing valvular lesion. The location where the vegetation in infective endocarditis develops is based on the pressure of the surroundings. Bacteria tend to colonise places where the pressure is low.
'Where the pressure is low', depends upon what kind of a valve deformity is already present.
In mitral regurgitation, blood when pumped from the left ventricle enters the atria due to the patent bicuspid valve. Naturally, the pressure is more on the ventricular side than the atrial side. Hence, the bacteria settle on the top of the leaf. Moreover, the do so at the edges of the leaves. This is because, since the blood has to squeeze through, according to to bernoulli's theorem (Venturi effect). Hence they form on the edges of the atrial side of the valves
In case of an aortic regurgitation, since there is a backflow of blood from the aorta to the ventricles, the vegetations are found on the underside of the aortic valves.
Liebmann Sacks (which is non infective and happens in lupus) is common in the mitral valve though it can happen anywhere. I assume, since bacteria are living cells, the pressure constraint applies to them but not to sterile vegetations like the one in lupus.
I implore you to have a look at this wonderful video which explains it all.
Picture credits and reference : Endocarditis by osmosis.org