As for this part, Dick Johnson gave an interesting talk about it at a
safety
talk at the Region 9 Championships this year. He gave aerodynamic and
safety reasons to always use a small slip while thermaling. I can't
recall
all the details, but have been doing it ever since then and I like it.

Larry Pardue 2I

I used a little slip ( inner wing forward) before I was flying with
winglets.
Now with winglets it is different. Most winglets only tolerate max + or - 5
deg.
change in angle of attack from optimum. I try to fly now with the string as
closely as possible in the centre.
Udo