Ok, so I never read the manual, and mine was used so never go a manual anyway. lol.
I don't know why they say you shouldn't use toe in but its probably not going to break the truck, as long as it is a degree or two either in toe-in or toe-out. The pro's and cons of either are varied, depending on other factors like caster and camber, but for the purposes of this vehicle, we could (wisely or not) assume the following:
Toe-out: Will provide better corner entry, and stabilised cornering, but straight line stability will be compromised.
Toe-in: Will compromise on corner entry but will provide better straight line stability.
I have found with on-road, 1/10th scale RC, that on a twisty track, its better to have some to-out, it helps with cornering, but on long tracks with high speed sections, a toe-in will provide stability at high speeds.
In the 'real world' of racing and also karts, remember that toe-in and toe-out, you will add rolling resistance, the more of either you apply, so you sacrifice on a bit of power, and also create more wear on the tires. But, this is not real world, and if you are not racing, you would want to have as much stability as possible so you can enjoy the truck, I am sure that you would prefer to sacrifice a little cornering by having a truck that goes in a straight line, right?
You wont damage the truck. Just make sure you remove as much play as possible on the steering linkages, your toe-in is only a few degrees and your steering saver is functional, without being too loose. If its too tight and you hit a curb, say good bye to your servo. Too lose and you will suffer with bad stability.
I am not an expert on this, but these are my findings, and I find it very difficult to believe that you will do any damage with a little toe in.