Yes I have with positive and negative consequences. I prefer Style A but I have since forgotten exactly what this style sheet's fields are supposed to be since I customized it a few months ago. BEWARE...when I was customizing (by clicking the pencil at the end of Styles) somehow I accidently erased all the field definitions and I had to start from scratch. Now my Styles C, D, and E have no fields besides Shared and I cannot set them back to their defaults. I use only Style A and Style B with the difference being that Style A lists Reviews and Style B lists Entry Date, as well as Book Cover, Title, Author, Date, Tags, Rating and Shared.
I would say Reviews are my favourite customized item so far...I like being able to sort or search by author and see which books I have written reviews on and which I have not. I will have to check the other items that can be added. I seem to remember that we could only have 8 or 9 different fields per style sheet but now it looks like you can have up to 12.
I hope LibaryThing one day adds a feature to set style sheets back to defaults.