T1C, I am a project manager for a roofing company and I bid work all the time for people who are interested in planning for future projects. I can't speak for any one else, but I don't mind giving quotes for people to be able to set a budget either for immediate start dates or delayed start dates.
Don't worry about ticking off the contractors, cause we work for you, not the other way around. Find contractors that will educate you on the installation of their trades and can work within the definition of the scope of work that you detail. Remember that pricing is volatile and can change as material and labor cost change.