Mashing in the upper 150F range will still produce fairly high fermentability. In the case of a Mild, there is not a lot of sugars in the wort so you will generally want to tailor the mash to improve mouthfeel and residual sweetness in the finish.
I would not consider performing a step mash for this style. I've only brewed about 20 batches of Mild though. It is one of my favorite styles.