I assume you're talking about running boards when you say steps.
If so, a better way would be to measure the overall diameter of the wheel (or rim) +tire. When you say 18" or 22", you are referring only to the rims and are not accounting for the tires. Generally larger wheels have skinnier tires and vice versa so that the overall size remains roughly the same. Unless you go aftermarket wheels and tires, overall tire size from factory will not make much of a difference.
You also mentioned a new truck. If you're considering a new truck overall, there are many more factors to consider to include new body style, trim levels, and different suspension setups.
**For example, an AT4 Sierra is going to ride higher than a SLT. AT4 has 18" or 20" wheels but overall tire size will stay similar. The suspension will play a larger part than either of the wheel selection that comes from the dealership. **On the opposite extreme, you've probably seen the Jeeps with massive tires on them. They usually run on 17" wheels but the tires are much larger in comparison and will likely play a larger role in vehicle height than the suspension (or a combination of both) that will make getting in much more difficult.
All this to say, it's hard to say without more info.