IMO, http requests are bad for your app as they tend to be a
bottleneck area depending on the concurrent requests a browser makes
(older browsers do only two connections at a time for example). But
with modern browsers you can count on them being smart and loading the
same image from the local cache instead of hitting the server. So in
general terms you could probably not worry about it and never see
unneeded requests.
That being said, you end up counting on "magic" on the browser's part
and would be better off ensuring your app only calls Asset.images once
to preload the images and then just cloning the resulting image
elements as needed to ensure no unneeded requests are ever made.