Your guiding principle should be that whenever it is possible to find out the name of the appropriate individual with hiring power, that's whom you should address your letter to. The only time it is acceptable to address your letter to a title is when there is absolutely no way to find out the name of the person with hiring power for that job.

The ideal person to whom to address your letter is the person with hiring power for the specific position you seek. The second best choice is the president of the company or another high-level executive who oversees your area. This high-level executive will likely pass your letter on to someone else, but the someone else will give your letter more attention because the honcho executive asked that the letter be handled. The last choice is the personnel or human-resources department. That department is not the best destination for your letter because its job is to screen applicants out. So, try to get the name of the hiring manager for your ideal position, but if you can't, obtain the name of someone else with hiring power.