This is an advice which is actually not a mixing advice, though it works quite well as a mixing tool: If parts of your sample-based orchestral composition don’t sound right, like sharp or unrealistic, don’t EQ or compress, try fixing it with orchestration first, with doubling instruments being the simplest method.
The composers of the pre-digital era used orchestration not only as a means to create incredibly varied soundscapes, but also to make sure the performance sounded balanced and appealing to the ear. If the piccolo hurts, try doubling it with flute or violins. If the oboe sounds too nasal, double it with clarinet. If the clarinet lacks edge, double it with bassoon. And so on. If you can, listen to some John Williams stuff and focus on the Celesta. It’s surprisingly frequent, not as a lead instrument (Harry Potter), but as a colorful “add-on” that for example adds subliminal substance to delicate violin figures. In the world of samples, the Celesta can also be used to mask not-so-perfect attacks of strings. You get the idea.
My approach: When I started midi-composing I always thought that every instrument I had used was to be heard clearly in the final mix. I’ve come to the conclusion that sometimes it takes subtlety to achieve the best sound possible. As of lately I try to become more and more like a painter, playing with sound colors by adding nuances that you might not be able to discern, but they’re there and they do make a difference.