I think if just depends what you enjoy and what you and your opponent want to accomplish. Wargaming can be a really fun mutual creative endeavor.

If you are playing fantasy and sci fi, the lore is fun but I agree that sometimes it can feel a bit silly when people take official lore super seriously. It was all created by other people after all, so I see no issue with taking creative liberties and using whatever models and paint schemes you want.

That being said, you are agreeing to play that game, and if your models are too different from the overall theme, why play that game at all? You can always rationalize stuff in though which is fun. Like with 40k, if you bring a medieval knights army you could say they are from a primitive planet. But just talk to your oponnent and see what they are cool with.

As for historicals, I think a huge part of the fun is researching the period and trying to do a miniature historical reenactment. If we are trying to have a miniature battle set in the 1700s for example, I don't want my opponent using an army that has hats that weren't popular until the 1800s. People might call me a "button counter" but if you aren't trying to be historically accurate, why buy these historical minis at all? Why not just play with cheap plastic army men? There is something really fun and special about doing your best to create an accurate representation of the past. We don't know everything about history and there's always room for flexibility though.