Thank for the replies. My question was definitely more about policy more than advisability. Just wondering if there was any restriction against including the web address of my favorite charity, for example. Or any other sort of commercial or non-profit enterprise. Looks like there isn't.... Thanks!
Based on my experience as an Android app dev, I have to agree w/ Matt & Steven here. It's downright shocking how virulently opposed to advertising end users can be, even in apps that are totally free. Many an Amazon Free App of the Day has been pilloried in the reviews on account of ads, in-app purchase availability or totally harmless and non-invasive analytics present in the app. The majority of users seem to think any ad = spam, even when it's just that the app splash screen includes a tiny, unobtrusive link to a page where the same dev's other apps are listed---a page the user can completely ignore by NOT clicking the link. Too many users also mistakenly think any kind of analytics = spyware, so tread lightly on that one too, especially since the paranoid types are already out in force to label the Echo an NSA spying device. ='/