I am building up a set or 27" Weinmann concave rims, laced to black anodized Dura Ace high flange hubs. I have owned, and probably still do, 27", 700c narrow and 700c wide - all were fitted with eyelets. Do they matter?

Yes - they spread the load created at the nipple head. Without eyelets, sooner or later, and sooner is most likely, the rim will crack/split at the nipple hole, particularly if the spokes are not evenly tensioned. So, eyelets or forget abboud it, in my opinion. If you don't have a rim with eyelets, I suggest the nipple washers which are a bit of a pain to install but will do the same job as a rim's integral eyelets.