Forced air heating air inlet in crawl space
For heating I like to have an intake at the lowest part of the house to pull out the cold air.  This may very well be in or near that crawlspace.  If it has a filter, then I wouldn't worry about dust.  Just be sure you're replacing the filter regularly.  However, I would want to know how fresh air gets into that crawlspace.  If it's sealed off, then you are hurting the performance of your heating system.  Otherwise, your intake is effectively where ever the opening to the crawlspace is.

When possible, it's nice to have a short path between the return vent and the blower unit.  In addition to less cost of materials, you have less distance you are forcing the blower to suck the air through, and therefore more efficiency.