It depends on the matchup and on your sideboard plan. There tend to be particular cards that are weak in particular matchups, such as Petrified Field when your opponent does not disrupt your lands or Mindbreak Trap against a slower deck like White Eldrazi.
Beyond that you need to trim intelligently. For example, I typically cut Cabal Therapy vs Shops because if you are resolving a spell you would much prefer Dread Return. More generally you need to consider the speed with which you need to deliver a kill. Ichorid is typically a turn slower than Narcomoeba and half a turn slower (on average) than Bloodghast in terms of putting a body on the board, so you want to trim them somewhat in faster matchups, but it is invaluable in slower, more grindy matchups. You can consider cutting the whole Dread Return package in slower matchups, that's an easy way to free up a number of spots in the deck. Typically the faster the matchup is the closer to your main-deck configuration you would like to remain.
Against Oath in particular their graveyard hate is a little weaker; they can't run Cage for obvious reasons, so they are typically dependent on opening-hand Leyline. You can often decline to put a creature into play until you can present an unbeatable board in a single turn. Practice and experience will be better guides in that type of matchup than anything we have to say.
I know some people trim Serum Powder while sideboarding, and I've done it myself in very particular circumstances, but I think it is usually a mistake when others do it and has proven to be mistaken even in the few instances I have done it.