Do I still need to add and worry about keywords for managed placements? I mean, if I want my ads to show there regardless, seems like having keywords to match my ads to relevant sites on the content network isn't an issue, but maybe there's another reason? I mean, how exactly do keywords relate to managed placement on display? Thanks.
No. Keywords only defeat the purpose if you use managed placement. The only reason you'd use both is for example, targeting gmail accounts but want to show your ads only when the email matches your theme. Otherwise, don't mix the two.