Filling holes in classic US sets is a tough registry task.
I suspected that the change in policy by NGC to not allow PCGS coins would make registry participation harder. I primarily collect classic US coins and was just finishing up an SLQ one-per-date set and an 1890-1909 Indian Head Cent Set.
With just a few holes left in each of these sets, it seemed relatively certain that I would fill these shortly and could start thinking about which series I'd like to pursue next. Most often I have to consider key dates, the grade range I will shoot for and whether I will collect an entire series or break it up as a "short set" ( like I have done with the 1900's Barber Halves, my Indian Head short set of 1890-1909, Walkers, 1930's Buffs and Mercs.)
However now I have to also consider if the coins simply exist in NGC holders. As an example, my Standing Liberty Quarter one-per-date set has been named "The Almost Set" as I made the criteria for AU grade coins only. A quick perusal of auction histories and Ebay will show that 80% of SLQ's for the final 3 holes I need are in PCGS holders---and it may be just as high for Indian Head cents in the MS64/65 grades. I have looked at the few that have been available in NGC holders and they just didn't have the nice PQ look to them for the grade that I have been assembling ( although they appear to be technically graded accurately for the most part.)
What is more frustrating is the fact that I have seen an acceptable PCGS slabbed coin that would fit. I even bought a nice 1919 SLQ that was too nice to pass up. This leaves me with the choice--move my sets out of the registry and into the wastelands of the seldom-viewed "Custom Sets" so that I can complete them with the best coins or play the long game and eventually find the final few pieces in the proper, acceptable plastic holder.
It is what it is and I usually just remove the classic sets from the registry as I complete them anyhow ( except for one set which I leave that won a presentation award).
As an off-topic aside, I renewed my membership in November at the Premium level to get the $150 credit for submissions. I have LOTS of coins that have been waiting to be slabbed for quite a while. In early February it dawned on me that I had not received my membership renewal pack in the mail. Now I know that we no longer get a free coin in an NGC slab, a nice NGC pin, notepad or pens and classy folders with glossy paged price list and submission forms anymore, but I thought we still got the envelope with a welcome letter and coupons. I called NGC and they said they don't send anything anymore, that when you join or renew that you just get a credit when your submission is sent--not even a thank you or welcome back anymore. I guess I'm getting to be a dinosaur and the impersonal businesses of the 21st century are the new norm.
Here's my 1919-S SLQ AU55---thinking of starting a custom set called, "The Isle of Misfit Coins" for the nice PCGS coins I buy.....