Is heat output a important factor in building a computer.

dust is important, if dust is clogging your vents/heatsinks, cooling is drastically reduced and fans need to work harder. Anyone who builds there own system would be dusting frequently if they're system is in a dust prone room. Even more frequently if they tamper with OC'ing or have a windowed case to show off the internals.
Though unless you have big holes/slots from where dust can get sucked in, it will take a while for it to build up to a point where it affects cooling.
Then there are those that build a system and don't care, leave it to accumulate till it's visible and noticeably affecting cooling performance.
But these are super-fine dust particles in question, so it will be many months in most circumstances, before it may seem worth cleaning, my dustiest tower is an antec silent case which is fine for half a year in the loft.
Dust will be an issue in both -ve/+ve,
IMO, choose which provides better cooling and have a routine of cleaning dust regularly.
I use can of compressed air ($12 for 4x 500ml) to blow it out and hold a vacuum hose nearby for it to be sucked away rather than circulate the room, only to be sucked back into the case quicker
If you want a better dust solution, find the major places your case will allow dust through and have dust filters fitted. Example> http://www.frozencpu.com/products/16427/ffi-102/FrozenCPUcom_Premium_Dust_Filter_Material_500mm_x_400mm_x_3mm.html
When you turn your pc off, cover it up with an anti-static material cover.

To put my perspective of negative flow visually, think of a case where smoke is used to represent heat. With positive, the smoke will be circulating round longer before it is exhausted. With negative, it is exhausted quicker for intakes to bring in cool air and circulate that cooler/'non-smoky' air, aka,heat is dissipated faster than cool comes in rather than coll coming in faster, mixing with hot before its exhausted (to an extent, just being visually descriptive)

Both types of airflow are favoured, seems personal preference, Link mentioned previously to other post validates both are used between 680votes.
Go for whichever you prefer. Though ultimately, there are tiny differences in performance and what will matter most as long as it's a decent airflow case with filters in the right places, is the performance of the individual cooling components.