Do you think that 5% is enough, maybe 10% or sth else?

I recommend something else.
That is: 0.

From my limited experience, 0-3% never helps, except for artificial tests I conducted myself.
I think that far less than 3% of downloads gets corrupted, therefore even 3% is a serious waste of bandwidth.

If you really care, I can recommend external par file with 10-15%.
It:
- is optional, so nobody who downloads w/out corruption looses bandwidth.
- is far bigger than 3%, possibly big enough to help in most cases
- is significantly smaller than the archive, so it makes sense for people who had the file corrupted
- is more secure than rar with the same amount of spare data
- takes more of your bandwidth (comparing with 0%, 5% or alike)