If I am not earning money (i.e. being paid in the local currency) I do not get a visa. If I will be earning money I get a Visa. That said when I enter I make it clear that I am entering on business and not being paid. In my case the vast majority of my trips are to attend a conference or to work with research collaborators.

Only have have been asked about earning money once and that was entering Canada. That local organizers sometime pay my expenses but that is a reimbursement.