I agree with Dunster's answer, although I don't always follow best practices on family quilts (trying to be better about that).
If you trim earlier, you will need to re-work your math across more steps unless every single time you're trimming it's because your block ended up larger than what it was supposed to be. The upside of that is that once you do the math, you're likely to end up with blocks that are proportional & square as long as they're pieced properly. The downside is that you are spending a lot of time between cutting every single block to the new target size & refiguring the sizes of the setting triangles & sashing.
If you trim later, you're likely to have crooked blocks because some will be bigger than others (if I understand right. If they're all the same too-large size, the only issue will be that the outer blocks in the set of 16 will be smaller than the inner blocks once you trim a 16-patch). The math will be easier to calculate since there are fewer steps to the end, but if there's a problem with cutting/piecing that is causing the blocks to not come out a consistent size, that will be compounded by piecing together pieces that are not the right size to start with.