Agree with what has been posted above. To me, though, it is less about the number grade that an off-brand company puts on the flip and more about the question of authenticity and/or alterations to the card.
Granted, no card companies are perfect, but the 3 listed (SGC, PSA, and Beckett) have a lot better chance at getting it right than others. GAI would be the 4th to add to that list, but their grades are commonly assumed to be 1/2 grade too high when compared to the other 3. Each company has their strengths. SGC has the firmest handle on prewar, PSA seems to hold down the post war market, but has some credibility in prewar and modern as well. BGS is king of the modern, shiny stuff. Beckett's vintage grading (BVG) is making progress at breaking into older stuff, but they are not as prevelent as the others.
Again, the biggest fear that I would have with an off brand is slabbing a card that isn't even authentic. The second fear is that it is trimmed or altered in some way. The distant third concern would be that it is graded incorrectly.
...and there are times that I would even fear trimming or alterations on cards graded by the big 3 - depending on the card. Remember that there are instances where guys have started slabbing cards in their garage. The industry has largely accepted those 3 (or 4 with GAI) as the ones with credibility. For me, personally, I much prefer SGC - not only due to the appearance of the cards in their slabs, but due to my respect for their understanding of prewar (read: more scarcely seen) cards. I largely collect prewar, so SGC is my strong preference followed by PSA and then the other 2.
As Leon said though... it is important to know about the cards you are buying before blindly trusting anybody. Good luck!
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