I agree. I have seen it work well both ways. I would see if I could determine whether the soils on the vehicles are alkaline or acidic. You should be able to find that out with a pH test strip. I have found that usually if you use the product first that matches the pH of soil on the vehicles works best. If acidic go with the low pH first. If alkaline go with the high pH first. This is not an iron clad solution but I have found it to work in most cases.
Some do switch seasonally starting with the low pH in cold weather and high pH in warmer temperatures but personally I think that is more of a preference on their part than anything else. If you live in an area with a bug problem you should probably go with the high pH first to help soften them.
Another factor is the strength of the presoaks you are using. It would be best to check with your supplier for that information unless you are skilled at titration.
In either case, heating the presoaks also increases cleaning ability.