Tip based on the service provided. If your wife believes adamantly, that the job that was completed wasn't done well, then don't tip well.
I always tip 20%, and try to tip at least 15%, even if the service was bad, because I'm a server at a restaurant.
For car transport, you should at least tip. If the company is paying for it, they aren't providing the tip either, assuming that's based on your judgement call.
Usually for manual labor, depending on the care provided for your belongings, and whether or not everything was set in place. Some movers have a company policy that if some fragile things are broken, they will not be replaced, and the moving company isn't expected to reimburse the cost either.
Generally, if the service provided was excellent, I would tip above 50, maybe skip the pizza, and just give them cash. Unless they explicitly ask for it.
I always have had issues with this beforehand, but the general rule of thumb is anything involving manual labor try to at least go around past 25%. Its pretty tough physical work, and there may be some mistakes involved down the line.
EDIT: People do get paid for stuff like this, but they don't get paid extraordinarily well. I worked at a movers company, but had to opt out due to how much stress it put on my back, and only made 13.00 dollars an hour, which isn't that bad, but considering you're only getting the hours that people request you for. I would work 20 hours a week, and make around 200-300 depending on tips. On a very busy holiday week where people are moving around more, I would walk out with 600-700 with the tips included, and it definitely helped my income.