Regardless of temps, get there early and fish it hard in the mornings. Once the spring time hatches wrap up the best fishing is generally from sun-up until 11:00 AM or so. Expect a mix of different sizes and colors of caddis, blue quills, midges, and tricos to varying degrees depending on location. Slate drakes will also start to show up in good numbers as the summer goes on. With the exception of some slate drakes, go small, even with the caddis.
There is a lot made of not fishing when the water is warm. It's pretty simple for me, if it's hot, the fishing is going to suck. So I don't bother. That said, if daytime highs are in the low 80's and night times lows are in the mid 60's or lower, I would not hesitate to fish the LJR. On, the other hand if it's been in the 90's for a week, you are probably going to have a crappy day.
Kev