We have two heavy duty plastic mounting blocks, one is a three step, the other is two, and we also have a four step ladder. I almost always use the three step one, it feels the most sturdy to me and the height is perfect. I really dislike the step ladder, it is kind of wobbly and it freaks me out. I'd hate to tip it over or have either myself or my horse get hung up in it somehow. It's just not safe.
I believe that it is better to use a mounting block or anything that makes mounting easier, it's healthier for my knees and back and for my horse's back. Also, I absolutely have to stand on something to get on my horse. I'm 5'3" and my horse is 16hh! There is no way I am hauling my butt up in the saddle from the ground.
I agree that your horse should be okay with being mounted from the ground, but I think it depends on what you do with your horse. If you trail ride or compete outside of an arena, by all means, they need to be used to it. But if you're like me and spend 99% of the time in an arena, all you have to do to get back on is walk back to the mounting block!