Does giving an employee the same job on each game decrease their effectiveness?

I just bought Game Dev Story, and I've made a few games. Each time, I assign the developer with the best relevant stat to each task. For example, I always use the developer with the highest Scenario stat to write the proposal, the one with the highest Sound stat to do the music, etc.

However, around my third or fourth game, these people started saying "I'm doing the ____ again? I'm not sure I can give this one my best..."

Does this mean that assigning them to the same job each time causes them to lose effectiveness? Should I be occasionally hiring outside consultants or using new hires?

It appears that this is true.

When I play the game, I try to have multiple employees with high numbers in a certain stat (I have 2 high level sound people, 2 high level graphics people, etc). As you get farther into your companies career, you'll be able to hire hackers and other people with very strong skills across the board, making this even easier.

Also, when choosing somebody to work on a job, make sure to note the Prev symbol that appears to the left of In or Out. The Prev means that that person has done that job on the last game you made - this allows you to cycle people without having to remember who worked on what last.