"Tjerk
Yes and no. It depends on how many races a week you have, how long the season break is, how much you care about your manager ranking and if you want to be behind the field every other season.
1. If it's an every day league with a short season break, high chances you will encounter managers with 100% cars from the first race on. If you start out with an average car it will likely take you untill the end of the season to develop a 100% car and also go for an average next seasons car. In those situations it's either accepting you will never win a championship or go along with the flow and go for on-off seasons. As for manager rankings, the off season will hurt it badly.
2. If it's an every day league with a long season break, lets say 6-8 days, you still might encounter managers with 100% cars from the first race on but you can get away with an average design every season. During the longer seasons break you will have the chance to first focus a bit on next seasons design while your current seasons design is on average and designing still goes pretty fast. After a couple design rounds you switch to the current seasons design and while you're still behind on the 100% cars, you're much closer and it will take less time to catch up. It might hurt the manager ranking a bit but not as badly and you will not be behind the field every other season.
3. If you're in a league that does 3 races a week, for example, you shouldn't have to think about going for on-off season strategies. There will be plenty of time to get an average next seasons design and because of the lower amount of races, there will be time to focus on getting this seasons car going and next seasons car towards it's usual average. There still will be managers starting off with 100% but by the time it's halfway the season, things changed dramatically as also the ones starting out on an average design will have caught up. When drivers properly trained, the 100% starters can be beaten for the championship.
Nice!