Opamp non idealities
Offset voltage is unavoidable error in the construction of a opamp.  Nobody "places" it anywhere.  Manufacturers go to great lengths to reduce it, but of course, can not make it zero.

It also makes no sense to ask whether the offset voltage appears on the positive or negative input.  The offset is between the two inputs.  A opamp ideally does:

&nbsp;&nbsp;Out = Gain(Pos - Neg)

In reality, it does:

&nbsp;&nbsp;Out = Gain(Pos - Neg + Ofs)

The offset error is not specifically added to either input.  It is a error resulting from finding the difference between the two inputs.  You therefore can't say whether it appears on just the positive or just the negative input, and you wouldn't be able to tell the difference anyway.