I used to collect my Wilhelmina perfins the way I suppose many others do: on used single copies.
Although this is an easy way to collect them, I've since found that it is much nicer to try and collect them on cover. And especially on covers which have the name of the company printed on the envelope.
That way, the whole item not only shows that that particular stamp comes with a perfin, but also which company is represented by the perfin, something which may be hard to find otherwise, unless of course you belong to a perfin club! I must admit, though, that the perfin itself does not show up as nicely as when you mount it against a black background, but if you study the stamp a bit more closely you'll see the perfin JTV / &Co easily enough!
Although I must admit that I cannot read the perfin on my 10c cover, even though I know it is sent by NV Hartelust's IJzerhandel in Leeuwarden. The postmark is so heavy that I cannot distinguish the perfin at all. maybe I'll have to be on the lookout for a better example! And I know that the whole thing takes up much more album space, of course, but as I'm not going for perfin completion here, but just a copy of each value of the set on cover, that's not an unsurmountable problem.