I agree totally with Jeff .
A couple of years ago when I started digital printing I didn't use profiles and wasted heaps of media and time tweaking to get the results I wanted. Profiles changed everything, enabling production of very good prints straight off . Often small adjustments then produced prints of great beauty, consistently better than anything I could have done in the darkroom.
There are a couple of important provisos.
The profiles must be built by an expert who not only does the correct thing technically, but knows how to make beautiful profiles of great luminosity and depth. There are a lot of lousy profiles out there . Just paying someone $100 or so does not ensure great results. Sensitive profile editing is essential.
Secondly soft proofing must be done in correct viewing conditions. That includes low ambient light conditions and the viewing of the print with a full spectrum light source at the correct colour temperature. Again Jeff's advice in right on.
All this assumes calibrated monitor of sufficient quality, calibrated scanner if one is used, and use of an appropriate colour space and rendering intent.