I'm going to answer with what you want to know, and not what you're asking. Bear with me.
FSB Mercs are not uncommon, except with certain issues and in certain grades. The first thing for you is to determine which of your coins have sufficient value to warrant putting in to a slab, and by "value" I mean something greater than just how much money you can liquidate them for. You need to be comfortable grading them, knowledgeable of which issues are rare in FSB and/or Conditional Rarities, aware of which are more liquid in the marketplace than others (price is a combination of both rarity
and demand), and set on your long-term intent for these coins.
We're going to assume that your interest in slabbing relates to your concern for those who will follow, and inherit your holdings. It should be relatively obvious that you'll want your heirs to be aware of the value of these coins, and therefore will have specific language written somewhere (in your will, in a detail addendum to it, in a "hold until my death" letter, something). This is
far more important than having the coins in slabs. Your heirs will be just as quickly taken advantage of whether the coins are in slabs or not, if they proceed without accurate knowledge of what they have.
And knowing that, you'll be able to tell them which of your coins need to receive that extra attention - slabbing, bespoke auctions - when the time comes.
Wait for that time for slabbing. Task your Executor with the job. I say this because (to my mind) the handwriting is on the wall regarding the mid-term future of the slabbing industry. Already, they've exhausted (essentially) the collectable-coin market - pretty much everything old which deserves a slab is in one. They're now being held up by the less-than-expert market; the Home Shopping Networks of the world where "First Issue" propaganda allows them to leverage unknowing buyers into paying more than the true worth for coins which ought to just be spent.
This is not a sustainable business model. I do not believe the landscape will look like it does today, ten years from now. I do not believe there will be as many
TPG's then. Heck, the survivor(s) may not even carry the same name(s) as the current competitors.
Today, for most issues, PCGS is the slab you want your coins in, for their superior performance in the marketplace. However, that statement is not as true as it was five years ago. The field is leveling somewhat because we, the buyers, are (thanks to instant information dissemination via the Internet) becoming far better educated far more quickly than ever before. We have instant access to highly knowledgeable peers, huge high-resolution images and large databases of prior sales results. Numismatic decisions are being made in the harsh light of reality when before much was about rumor and incomplete information.
Don't slab your coins. Not yet. We can have no reasonable opinion of what the world will look like when the time comes for your eligible coins to be presented in slabs - or whatever the solution is, then, or who the 800lb gorilla in the field will be. The physical cost will go up by then, but one reasonable inference is that such cost will remain tied to the actual value of the coin - nothing else would be profitable.
You cannot make a decision today which will guarantee the best-possible outcome for those who will benefit from the decision. This isn't the world they will live in. The best you can do is to codify all possible information about your coins, ensure that it is
known that they will benefit from specific additional handling when the time comes, and trust those who follow you.