I like using Coats or Gutterman's all-cotton thread, if it's an all-cotton top that I'm piecing. I do this so the strength of the thread matches the strength of the fabric. Not sure that it's necessary, but that's what I've liked over the last few years.
The thread thickness can throw off your 1/4" seam ... you can adjust by doing a scant 1/4" if your thread is really thick.
But, the more important thing is that you are consistent throughout the blocks and all the piecing. That's what will get your seams to match.
It's another good reason to do a couple of sample blocks ... you can adjust your seams if the block size you're ending up with isn't quite right. And yet another 'but' - still won't matter if you're consistent with the seams for that top!