If you finish 6th and play 3 games by deliberately losing either first or second to gain more money from attendance, you will get:
1st game (home): Your arena will be full and you will get 2/3 out of it (1/3 goes to your opponent)
2nd game (away): Your opponents' arena will be full and you will get 1/3 out of it
3rd game (home): Same as 1st game
Now calculate how much are you going to get if you finish sixth (anyone could do it, looking at your and your opponents' arena, but I'm too lazy for that).
Then calculate how much are you going to get if you finish 5th and not pay salaries for two weeks, but also don't get any arena income.
Training:
If you finish 5th, you will be stuck with only 2 games/week for 3 weeks straight.
If you finish 6th and go for 3 relegation games, you will be able to play 3 games/week in first two weeks and only 2 games for 3rd week.
If you are training one-position, there is no harm done if you finish 5th since you can train either JS, 1 on 1 or RB in those 3 off weeks.