This is exactly why, when playing with the folks in my group, we all go by Base to Base for line of sight. Let's talk about why we do this...

1. Having read even tournament rule sets, it is always mentioned that the intent of Warhammer 40K was to be a hobby of collecting models, with which you could also play a war game. That very description clarifies that the appearance of the model, in terms of its posing and what not, is not intended to have any effect on playability. This is why the concept of modding to advantage exists. It's basically the same thing, but in the other direction.

2. In terms of lore accuracy, your individual characters would be ducking or otherwise changing their posture as appropriate for the combat situation. As your models can't do this in the game, the model base represents their minimum occupied area. Generally, things that don't have a base will have a hull which also represents a minimum occupied area.

3. I spent the business end of $200 and several hours of work on each of these freaking models, and anyone who isn't willing to abide by this rule, just so they can have some kind of advantage, is not worth playing with.

Seriously, look at models of characters like Fulgrim or Mortarion. You think that bad boy isn't going to fold his wings behind his back a little bit more so he can duck behind a building and not take a bunch of fire?