APPLY TO BLANK SLIDE, REASON #1: There’s a small bug in Producer (currently v.4.51.3003) that will likely be fixed soon, but until then, you may be better off in certain cases applying slide styles to blank slides and only after that filling in the empty placeholders. The bug pops up in slide styles where a layer is duplicated and at least one of those duplicated layers sits inside a mask. This setup turns off the automatic behavior of duplicates, forcing you to seek out every duplicate so you can change it manually if you decide to switch the image to another one. But if you apply such a style to a blank slide, then fill in the placeholders, the bug goes away, and as many times as you change your mind about which photo to use, all duplicates will change too.
APPLY TO BLANK SLIDE, REASON #2: When a style involves a series of photos or specific positions for photos, if you want to control who goes where and when, applying the style to a blank slide makes photo timing and positioning a snap. Just select the photos you want in each photo layer. It beats juggling photos.
In most cases, it doesn’t matter which way you do it–add photos and then the style or add the style to a blank–but if you see things aren’t changing automatically as you’d rightfully expect, delete the slide, apply the style to a blank slide, fill the placeholders, and all will be well. As for a multi-photo style, you have complete control when applying it to a blank slide.