From personal belief and experience, this topic consists of a few issues..
The first thing you should do with the trainer who tells you to stand up straight, is ask him why he wants you to stand and fight this way.
If his response is something like "because I said so and know what's right" then maybe he doesn't actually have a clue.
If he gives you a detailed response of how it is more effective for a person like you (because of eg: height, body shape, speed, strength, etc) then he will probably know what he is on about.

The second thing is how comfortable you are in that stance. Give it roughly two weeks to adjust. Train standing upright for two weeks. If at the end, you still feel uncomfortable, then it's probably best you don't train like this.
You can't expect a boxer to fight in a stance if he doesn't 'feel it naturally'.

Thirdly, test yourself against a sparring partner. Try 2-3 rounds upright and then 2-3 rounds in the stance you feel comfortable with. Which stance has the smoother motions? Which stance gives more stability? Does upright send you off balance when you are hit? Does bending give you more power?

Personally I do not practice having on stance in a bout. I train and fight, moving and counter-attacking the oppenent. If the other guy is standing upright, I'm gonna go in small and weave, landing body shots. If the other guy is fighting small, I'm going to go upright, on my toes, and throw the head shots and uppercuts into his lowered head.

Learn how to fight against each style of fighter, learn well. Then you can react to each boxer differently. Even if you are tall, like me, you can still go in small. Ducking and bobbing all over the place. Workout out your legs muscles.

Like a mentor, Cus D'Amato, once said. "It's good to throw the punch where you can hit him, but he can't hit you."
In otherwords, if he is fighting small and low, you should fight tall and stay arms distance away, working the jabs and straights.
If he is fighting tall, get in real close and land the hooks and uppercuts with body shots. It would be harder for an upright fighter to hit a close-small fighter.

Act according to your oppenent.

Peace!