@Jim472, yes, you're allowed 3 IB cancellations per year, but it sounds as if you're speaking about a regular request.
For regular requests, it is indeed very frustrating when people request to book and they don't meet your requriements. This is one of many glaring loop holes in the AirBnb system, which puts hosts at a disadvantage and can make it difficult to meet the target goals.
For the 88% acceptance rate, a while back I dropped down to 92% acceptance, partially because I had to deny a single guest twice. I was a bit concerned, so I contacted Airbnb and was told that maintaining the 88% acceptance rate is not as important as maintaining the 100% response rate.
Of course, we should keep the acceptance rate as high as possible, but dropping below that will not damage your Superhost status. In fact, we've had several other posters here who have claimed their acceptance rate is well below that and has been for a long time.
That said, it seems likely that declining too many requests will result in placing your listing further down on the search results.