Ian came before Toby, and was in fact the very first reincarnation of Bill since the Transcendence. He was a decent enough guy, but he wasn’t perfect – so Dipper was sure it was a fluke, that Bill’s soul was usually just as corrupt and terrible as you’d expect it to be.

So when Toby came along, Dipper thought that this would finally be his chance to prove it. Of course, you can imagine his frustration upon realizing Toby was even more of a sweetheart than Ian.

Despite having a fairly good track record, there were still times with Ian where his past life resurfaced a bit (such as calling R!Mabel “Shooting Star”), so even though Toby continued to be a precious little angel, Dipper had good reason to continue being wary of him. 

(For the record, Bill never really manifests on Toby in any significant way; he’s sometimes plagued by nightmares that are really visions of his past lives, but that’s about the extent of it.)