It is probably true that on half the home inspections I do the filter is installed backwards. It is important that the filter be installed the right way! Why? Because one side is stronger and meant to handle the air flow's pressure. There is an arrow drawn on every filter to tell you the air flow direction. Put in backwards, and as it gets loaded with dust, the filter can literally get sucked into the system and even cause damage to the fan.
Someone took the time to draw which way the filter should be installed.
That way, when it is put into the slot, you only have to put the arrow on the filter in the direction of the arrows drawn on the return duct.
What if the arrows are drawn wrong?
THESE ARROWS ARE POINTING IN THE WRONG DIRECTION!!
How do you know which way the air blows?
The air will always be drawn TOWARD the blower. From the blower it gets heated in the furnace or cooled on the AC coils.
So the air-flow arrows on the filter should point TOWARD blower.
This unit is 5 years old. It was filthy. The ducts in the house were filthy. Could it be the bad information on the arrows contributing to that?
My recommendation: If you don't know which way the air is blowing, put a piece of toilet paper on the end of a pair of pliers and stick it into the slot. See which way the paper is drawn. That is your air flow direction! Don't believe me? Check it yourself. AND THEN DRAW YOUR ARROWS THE RIGHT WAY!