Are saddles supposed to bow inwards when sat upon? One common theme I've noticed among saddles is that when I sit on them, they bow inwards at the middle where there's no support from the rails.

In order to compensate for this, I find myself tilting my saddle back a bit farther than I'd like it to be, so that my weight stays on the sit-bones. If I don't tilt the saddle back, I end up sliding forwards.

I once had a really horrible BSO saddle that suffered heavily from this effect. When I sat on it, it basically became a banana. I ended up jamming a block of wood between the rails and the bottom of the seat, and it fixed the issue. I've since upgraded to a better saddle (Velo), but it still does a bit of this.

Is this supposed to happen?

Edit:

I ended up solving this buy moving my saddle back. It kept me from sliding forwards and allowed me to nose the saddle down to level.
Yes, this is how saddles work. If you find yourself sitting on the rear edge of the saddle and sliding forward, you probably need a seatpost with more offset.

You might also be interested in SDG I-beam design. It has been around for more than 10 years and while not unpopular enough to be out of production, it also hasn't replaced the traditional rails design.