Can I Trust PCGS Holders If I Go To A Coin Show

Short answer is Yes. If your concern is counterfeits, both have industry-best detection methods. Accurate grading is another matter and subjective.

In the past both have holdered counterfeits. I wished I would have purchased a few. PCGS mistakenly authenticated the small 'O' Morgan dollars from 1900, 1901 and 1902. These were privately made and were discovered to be counterfeits less than 10 years ago. Those that made it into PCGS plastic are popular with collectors and priced accordingly.

While a lot of people have said that fake PCGS and NGC slab are commonly encountered at shows, I've run into very few of them. Of course 'm not able to attend the really big shows, maybe they are more common there. But as for coins in genuine PCGS, NGC, ANACS, SEGS, and ICG slabs not only are counterfeits in these slabs extremely rare, but if one does turn up the company will reimburse you for it. So you run no real risk.

As noted your fairly safe purchasing slabbed coins by the big named TPGS's. However, not only are there faked slabbed coins in faked slabs, but some really good people know how to open a slab, change the coin, reseal the slab so it is hardly noitced. Yes a close inspection by someone that knows slabs can possibly see where it was done, but not as much as people think. I've been shown slabs that were opened and the coin replaced as a joke and asked if I could see where it was done. I could not. The one I saw said it was a Cent but it was a Dime. I go to about 2 to 4 coin shows a Month and never sure about lots of things but it pays to ask.
One of the great things about coin shows is if you go to them often enough, you'll get to know some dealers that are more frequent. If in doubt about a coin, a slab, etc., just ask them. Sort of like having a second opinion about an operation. Nothing to loose by asking.

@just carl: Yes, my fear is that someone can open and reseal a holder, but, as you say, I'll trust someone and ask if I'm suspicious.
@BadThad: I don't go to that shop. I go somewhere else and will continue to do so. But, you know, this other coin dealer I met is what you don't want when you go into a coin shop. It's downright insulting. I can appreciate what I have where I normally go.

Yes, I'm learning this. It takes a while to catch on but I see that my MS64s are not equal. I don't know why standards vary like this. Isn't there a three-grader panel that decides the grade of each coin?

just carl's last comment are my sentiments as well.

Everyone is very helpful and I am grateful for your help. I learn a lot here.