For many brews, having trub in the fermenter is not a big deal, especially if you rack after a week into a secondary. The exception to this would be light or 'delicately' flavoured beers. Blonde ales or lighter lagers would be good examples. The risk is that the trub will begin to breakdown in the fermenter, and leave by-products that cause off flavours that will be most detectable in these types of beers. But it is only a risk, not a sure thing.
On the other hand, trub has a lot of good things in it that help initiate fermentation, so you don't want to be over-zealous about straining it out, either. A general rule of thumb is that a small amount of trub is good in the fermenter, and larger amounts can be tolerated by darker, heavier brews.