Every single midi that I have created has the source I used for the the music (and the source for any information) - whether it was full arrangement or single line meldoy. (I am a historian - I footnote because I consider my pages one huge research paper.) All but one or two midis that I have on my page has who sequenced it - there are only two that are anonymous (I could not track them down). I have contacted every single sequencer on my pages (two, I believe were returned with the e-mail address unknown). I have also put links to their personal pages or to their e-mail - for those that have a current one (some pages/addresses have died since).
The purpose of this is not just to give credit where credit is due - but also to expose people to resources they might otherwise miss and to other people's sites. When I first started folk music resources were extremely rare and I was glad to point people in other directions. I still am.
Several of the midis at the site have KNOWN sequencers who are not acknowledged. Brian Hick's has a copyright notice in his midis and he is not acknowledged. Barry Taylor did several and he is not acknowledged - though he asks that this be done when people use his midis.... Most knowledgeable sequencers embedd author information in their midis - which makes it very easy to find. If they've done so I assume they prefer to be acknowledged as its creator.
Share and share alike most certainly - but acknowledge those you benefit from - and give them benefit in return - whether it is personal pleasure they derive from having their work acknowledged and appreciated or by a link to their page.