We have a lot of honorarium payments submitted weekly. I get a lot of check requests for these honorariums without back-up attached. My company uses check request for anything we do not have a physical invoice for (deposits on hotels for conferences, postage for mailings etc.).
In general, it is a good business practice to get an invoice from all of your vendors. However, there are times where an invoice is not possible or they just refuse to submit one. For audit purposes, as long as you have the approval on the check request, attached to the check stub describing the services paid, then you should be okay. As part of your audit trail you will have not only this documentation but also the canceled check as proof towards the legitimacy of the transaction. It is important to have good internal controls over these types of payments as well. However, you should consult your tax advisor about your specific situation to make sure there is not something particular to honorarium payments that would require other documentation. If you can attach e-mail correspondence and any other documentation (e.g. for a speaking engagement, an agenda of an event showing the payee's name) that can substantiate the expense. This question is also a good one to post in the Messaging/Forum section of the site so that you can get feedback from your peers about how they are handling a similar situation and what seems to work for them in getting adequate documentation.