I think it definitely depends on your use-case. Like you wrote, if everything else you have open also churns through battery life, it's unlikely having Slack open will make a difference.
However, if your other applications are a terminal, other efficient text editor (e.g. Sublime), and other generally respectful apps, I've found having Slack open is the difference between having enough battery life to work all day, and not.
However, if your other applications are a terminal, other efficient text editor (e.g. Sublime), and other generally respectful apps, I've found having Slack open is the difference between having enough battery life to work all day, and not.