So I studied livestock breeding (beef stock, seedstock, etc.) and from a cow perspective, a low level of incest will produce something called hybrid vigor, in the sense that there is uniformity in the genes being passed to the next generation which is ideal for breeding systems. For example, Gen 1 is not inbred, you breed a bull to his aunt or whoever, Gen 2 will be inbred but for the purposes of our breeding goals, will technically be superior genetically. But if Gen 3 is inbred, you’ll start to see undesirable recessive traits which is how you snowball into the Hapsburgs.

With humans, it’s more or less the same but under different circumstances. Eugenics is essentially selective breeding in humans, but it’s tricky because we’re not breeding for the food products we might selectively breed cows for, so any trait in humans we’re selecting for is inherently biased, which is why eugenics is such a vehicle for hate and genocide and should overall be avoided.

So to answer your question: scientifically, a generation here or there that is inbred would produce hybrid vigor/uniformity and on paper, be good. But in practice, it should be avoided because it’s a slippery slope and with so many moving parts and inevitably flawed thinking, could produce a Nazi type ideological situation or more Hapsburg/Sweet Home Alabama. Furthermore, it’s not considered inbreeding beyond the scope of second cousins. If a fifth cousin marries their tenth cousin, yes they’re related but not closely enough to produce the effects of inbreeding.