I'm currently drawing a use case diagram (using UML2.0) and it's getting quite large and unorganised. So my question is, is it acceptable to place actors on both the left and right of the system boundary? I have seen several examples of this but I'm not sure if there are specific reasons why actors are placed on the right as opposed to the left? Or is there no difference really apart from design and aesthetics?
You can put them anywhere - the top, the bottom, the left, the right. In most cases, if it fits neatly, I've seen them on the right. They go on the left if necessary. UML doesn't specify this level of detail, though. You should make a diagram that is readable to the intended audience.