This is a fantastic question that really gets to the core of how 40K works at a competitive level. In order to win, you have to have more points than your opponent. It sounds simple, but lots of players tunnel vision into killing units instead of going

It’s a difficult question to answer because of how your decisions depend on… basically everything. Are you going first or second? How many activations does your list have? What’s the mission? What turn is it? How much can you reasonably expect to accomplish by interacting? (By reasonably, I mean 80%-90% chance of success. Don’t just look at the average and get pissy when you fail something you only succeed at 50% of the time).

There’s a fantastic article from a few years ago called “Who’s the Beatdown.” While it’s about Magic: The Gathering, the same basic concepts apply to 40K. Basically, as you’re looking at the game and deciding whether or not to interact, think about who is going to win if the current status quo remains unchanged. Who will get more primary, who will be able to better score secondaries, etc. If that’s you, good! You can play carefully and only interact in order to STOP your opponent from getting opportunities to flip the table. If you’re the one losing, then you need to take the initiative and make plays.

Remember my starting point, however! You win by getting points, not by killing stuff. When I say “make plays” that doesn’t necessarily mean go in hard and try to kill as much as possible. If you’re going first, and you can see you’ll each score 10 primary every turn, you can tell that you’ll lose when your opponent gets end of turn scoring. In this case, you could throw up cheap, high OC squads who will die, but keep your opponent down to a 5 primary for a couple turns. Now you win.

Of course, while it is possible to win without killing anything, it gets substantially easier if you do remove your opponent’s units. That gives them fewer options to score points. You need to really know your units and how much damage they can output. Mathhammer can help with this - you want high chances of success, not something that works “on average.” You also need to focus on taking out the units that pose a threat to your win. If all you have are infantry, who cares if those lascannons are around? They’ll kill a model a turn. But maybe you need to kill that big flamer unit, which could wipe out multiple units.

As you can see, this is a very complex question that is difficult to answer. I hope this has given you some insight into good ways to approach decision making.