Ok, for reference, I'm sure it's a combination of several things that is making my center a bit bubbly. Some I'll be doing differently in the future.
Cutting - I'm a newbie with the whole rotary cutting thing. I knew my triangles varied a tiny bit, I was just hoping it would even out across the top. It mostly did, but not enough!
Two things I think contributed:
First, ruler lines. I remember reading discussions on this board about "which side of the line" on the ruler to use to measure with. Couldn't figure out what that was about! By now, every time I cut I'm grumbling about how thick and wishy-washy the lines on my ruler are, lol! Since I cut those cheetah, my eye has developed to see more quilt-relevant precision.
Second, sliding the triangle ruler over, to cut two in one go (instead of swivelling it round and measuring and cutting painstakingly one by one). It's fast, Cute does it, it appealed to me. But for my personal skill (read: lack of it! lol) at this stage, it didn't give me precise enough triangles.
Aligning hexagons by pattern, not grain - yep I did. And will, again. I auditioned and re-auditioned each block, individually and in context with all the others I was cutting. This was a large part of the fun for me and I'll definitely keep doing it if I do more OBW. There has to be a way to do this and still end up with a viable result! Paula Nadelstern cuts the tiny fabric pieces for her super-complex kaleidoscopes every which way, looking only at the print and not the grain.
Seam allowances - I don't think this is a problem here. I pressed each seam open, from the reverse and then from the right side. (Another clothes-making habit.) So any bulk from seam allowances would be evenly distributed across the centre, not creating bubbly bits.
Pressing - possibly a factor. Although I've made tons of clothes and so have always worked with curves, bias and such (and yes I press, not iron, seams when I make clothes), so I don't entirely think I do a bad job pressing bias edges! I did find it hard to press enough to get the seams joining the strips entirely flattened out, so I'm thinking you ladies may have a point suggesting pressing with starch again.
There's a whole another saga beginning, with the starch. I don't like chemicals on my fabric and the commercial spray starch I could find locally stinks to high heaven. Yuck.
I've now found one that's supposedly natural, ordered it online... and it's going to take a week to be delivered and is costing me the equivalent of $15 for one spray bottle!! I sure hope it is good to work with, at that price. Well well, on to the next learning experience!