The whole point of racking it to a seconday is to get the beer away
from the yeast cake. This helps clear up the brew and reduces the chances for off-flavors caused by the yeast.
If fermentation is still going on strong when when you rack, you are going to get another
considerable cake of yeast forming in the bottom of the secondary as time passes, and this will defeat the purpose of racking it.
You will almost always have SOME yeast sediment in the secondary, but if you rack after primary fermentation has ceased, the amount will be minimal, and you get a cleaner beer going into the bottles.