Echoing Gerhard Paseman's comment more-or-less: it certainly never hurts to ask permission, or at least to explicitly express interest in use of on-line resources. Some on-line resources have indications of restrictions or lack thereof (e.g., Creative Commons License). In all cases, I think one should acknowledge on-line resources just as one would acknowledge traditional paper-media resources. Credit where credit is due.
Another point, more practical, is that the on-line versions may get updated, corrected, and revised, while downloaded (or printed) versions will certainly not. Thus, there is the "just in time" ("JIT") mechanism in play for on-line resources, namely, not to download (or print) until the last possible moment.
... and echoing anon, as well.