Large dust deposits are going to gather on the upstream side of the heatsink, so blowing air the opposite direction will have the best results in dislodging them**
** this hinges on the intake grille having enough open space to pass the dust bunnies OUT of the fan hub. Removing the bottom panel removes this restriction.
The [assembled] fan + heatsink channel is pretty well sealed off from the rest of the casing. Not perfect, but the majority of the air you blow in either direction should exit the casing.
Backwards. There's plenty of obstruction to airflow inside a laptop, and while it would normally make sense to blow in the exhaust to dislodge the dust bunnies from in front of whatever is collecting them, that just lodges the larger chunks on whatever the cleaner air of the intake must pass over. Like the heatpiping, drives, any chipset heatsinks etc. That's blowing dirt from the last 1/4 near the exhaust back into the first 3/4 nearer the intake.
Much care should be taken with compressed air in a laptop, it's extremely cold, and can/will form condensation almost instantly, the air coming out of the straw in an almost liquid form. Very short bursts, at most, and not on a running laptop or one that's been powered up recently. Best is to put it in the fridge for a few hours, away from the veggie draw, as fridges will not only chill the components ahead of time, but also remove a good chunk of the moisture from the air inside.
Seriously want to avoid condensation in any form. Condensation + dust bunnies = mud.
Best bet is regular blowouts, even weekly if around a dusty environment, and every 6 months-1 year pull the back off or whatever will come off, and give it a thorough blowout/dusting.