For sure, direct sunlight is the most-reliable approach to getting a good IR shot. I have only one counterexample and while most of the image is in the subtlety of the shade, there are still treetops in full sun there.

I don't generally go out with the IR film unless it's Sunny-16 conditions. I have a sneaking suspicion I should try it but I don't have an IR meter and want to get the most from my few remaining rolls of IR820 in a way that I know is reliable. Maybe once I get that 70mm roll of Rollei 400...