I don't think there is a single 'right' answer. Differing bounty amounts make for different types of tournaments - none right or wrong, just different.
$20 bounties for $80 to $100 buy-in events are pretty common (and popular, fwiw). So are $5 bounties for $20 to $60 buy-in events.
A close look at the two extremes indicates what type of play strategy may be needed to capitalize on the features of the structure:
A $100 buy-in event with all $100 going toward the bounty chip will best reward players who are tight-agressive, as merely out-lasting your opponents will not result in any cash winnings.
At the other extreme, a $100 buy-in event with only $1 going towards the bounty pretty much makes the bounty irrelevant in the big picture view.
So, to summarize..... a 1% bounty is irrelevant to play, and a 100% bounty is highly significant.
That said, I've found that 25% to 50% are the two extremes that make them both worth the added hassle and worth altering strategy to collect. But it's just an opinion. I've played in 100% bounty events that were a blast.... for some players. Some folks just don't care for the bounty concept in general.