Fan position doesn't matter regards dust concerns. There'll be intake one side of the fan and exhaust the other, and you need filtering on the intake regardless where it is. If we assume that the air is coming out the top, because we don't want air blowing down onto the desk/table/cabinet, then regardless whether the fan is at the top or in the middle or at the bottom, that top is an exhaust and the intakes are somewhere else, at the bottom or back or something. If these have filters, dust free. If not, there'll be dust. Alternatively, the exhausts are at the back, air's drawn in the top, and there's a nice filter on the top keeping it dust free inside.
In fact, the notion of positive and negative pressure systems is invalid. It's all about airflow and filtering. Air gets hot so has to be exhausted, and this has to be replaced or you'd create a vacuum which 1) ruins cooling and 2) would collapse the case. What we call a 'positive' pressure system just has a filter intake and whatever you like exhaust. You could just as readily switch that fan in reverse and add a whole bunch of filters on intake holes, or a single intake one side of the case with a filter and the exhaust the other and stick the fan on either- it should draw filtered air in and exhaust it at the same rate regardless.
