Never had a problem placing the filter to the far right or left on any of my tanks.
I would suggest though; IMO, If you do place it to an extreme left or right side you should do something to agitate the water movement on the opposite side since the feeding and suction are not centralized.
I usually just add an airstone to the calm water area, it helps aerate the tank anyways, so if your fish don't mind it (mine all seem to enjoy airstones) it does the job just fine.
A power head could also be used but on a 20gallon long you should have no problems.