An engine needs to be driven hard occasionally, meaning at freeway speeds and accelerated with a wide open throttle. Under such circumstances, engine combustion temperatures reach a peak and keep the engine clean by burning off deposits.
Hmm… I did not know that. What are those deposits to be burned?
Carbon deposits and buildup are a by-product of combustion in an engine that comes from incomplete fuel combustion. This black soot, similar to that which collects in a chimney, will harden on internal engine components such as injector nozzles, cylinder walls, intake valves, and more. Accelerating also tests when it is properly working, meaning there are less deposits.
I should be accelerating once in a while. 
Yes, you should as long as you are in the right place and careful in driving.