Well, it's not an obligation to participate. It's a choice. I usually only post a final entry when i'm fine with the theme and that I have time to put into getting a good pic.
Taking a few weeks of break can pay off too. I understand it can be hard to be part of the Top 20 each week, even if you submit great shots. Well, right now, pretty much everyone post awesome shots each week. Some of my best shots have never been chosen, but I take it positively, in respect to the other shots that made it.
Just for the fun of it, here's my backlog of the GT4 PhotoMode Competition up to now.
Week 1: Didn't participate
Week 2:
Participated and Picked (No Top 20 at that time) - 3 votes
Week 3: Didn't participate
Week 4: Didn't participate
Week 5: Participated and Not picked
Week 6: Participated and Not picked
Week 7: Didn't participate
Week 8: Didn't participate
Week 9: Participated and Not picked
Week 10: Participated and Not picked
Week 11: Participated and Not picked
Week 12: Didn't participate (Decided to take a break anyway)
Week 13: Didn't participate
Week 14: Didn't participate
Week 15:
Participated and Picked - 20 votes (WON)
Week 16:
Participated and Picked - 2 votes
Week 17: Didn't participate (Was theme picker)
Week 18: Didn't participate
Week 19: Participated and Not Picked
Week 20: Didn't participate
Week 21:
Participated and Picked - 7 votes
Week 22: Participated and Not Picked
Week 23: Didn't participate
Week 24:
Participated and Picked - 2 votes
Week 25: Participated and Not Picked
Between Week 2 and 15, I haven't been part of the Top 20... and then I get in on Week 15 and even win the competition. That, I think, was a really good pay off.
Anyway, in a total of 25 competitions, here are my stats:
Entries: 14
Top 20: 5
Wins: 1
Total Votes: 34
What counts for me is not to win absolutely, but to get at least one vote each week i'm in the Top 20. To me, it means that someone out there found it was the best of the bunch. Winning is just a bonus.