I have been told, but have never tried it, that conifers aren't the best for cooking - no matter how well seasoned they are there can still be strong creosote-type flavors in the smoke. If you're just using it to heat up the oven, I've heard it is OK, but they don't produce as many BTU's as hardwood. I prefer to have a live fire while I bake pizzas, so I use only hardwoods.
Bill/SFNM