I talked to a lawyer about this very thing. He specialized in visa work. Had his own setup and all. Over a phone call he said to me just to do all the paperwork properly and I should be set and that spending money on him probably isn't worth it.
His words were that if everything is done proper in paperwork, it's around a 95% chance of success in getting a spousal visa. He said hiring him would cost about 3 times the cost of the visa and raise it from 95% to 99%.
He said if we were looking for a tourist visa or an immigration visa or any of those things, that's where he is worth paying for. But for fiance, spousal, school/education, work and other such visas the success is more dependent on the those applying doing the proper job of just dotting every I and crossing every T and filling in all the forms to have success. All he'd do was double check that we did it all then show up at the embassy with us.
He did say if I wanted to be there with her throughout the process, I needed to petition for her. Then they cannot deny my presence in the interviews and the processes. If she applied, they'd keep me from much of the process.