I only know two things:
I saw a webcam picture back in Jan or Feb showing a nearly barren mountain.
I once climbed Shasta in August, and would not recommend that to anyone. (Views were horrible due to annual summer fires, too.) Compared to May/June ascents before, the glissading was short and instead was way more uneven cobbled broken basalt rock to descend on. It was miserable.
Based on the above, the sooner you go the better. Save it for another year if you can't make it by mid-June.
