Revenue-stamped checks are lots of fun, especially if you can find an interesting area to specialize in. Some people collect certain companies, other people collect vignettes (the graphic images engraved on early checks). Others look for certain towns and states.
Some people collect EMUs (early matched usages, e.g., a "bank check" revenue on a bank check, a "mortgage" revenue on a mortgage document, etc.). For the first few months following the introduction of the revenue stamp requirement in October 1862, you were only supposed to use the appropriate stamp on a given document. That was overturned very quickly, as people complained that it meant stocking too many different types and denominations of revenue stamps. After that point any revenue could be used on any document (for the most part).
That is what makes EMUs desirable.
I personally collect 1862 usages of all types on document, as they are fairly scarce. I have several October 1862 checks, the one below being one of the earliest usages on record.

Here's one on a check fragment that I *REALLY* wish I had the entire document for. It is R151a, one of the inverted centers.

Here is one of my more recent acquisitions, an R106b bisect on check. This is the original Scott Catalog listing example, as indicated in the accompanying letter. I just got a PF cert for it last month.

While this last one is not a check, it's one of my favorite documents. It is an R6e, which is the 2c Bank Check orange, but on green paper.
