A bore guide is necessary when cleaning with a rod.
Why?
Competitive shooter and Anschutz North America president Steven Boelter, who has written an excellent and detailed piece on rimfire cleaning, offers a good explanation. Preferring the term rod guide to bore guide because the tool guides the rod, not the bore, he says it "provides a guided path for the cleaning rod to follow, keeping it in alignment with the bore of the barrel, helping you to keep the rod from bumping into the inside of the receiver or breech face while working it back and forth. A good rod guide will also extend the life of your rod by preventing it from rubbing on the breech opening of the chamber or exposed lands in the throat of the barrel. These hard steel edges can quickly peel off any plastic coating on a rod or scratch a polished steel surface."
The entire treatise on Rimfire Cleaning by Boelter is well-worth reading. See h t t p://www.ssvtexel.nl/index.cfm?act=files.download&ui=C5C9D865-2200-0A21-B5F5CF897974784F
Bore guides (or rod guides) are often made of delrin, such as those by Possum Hollow. Some are made of aluminum, such as those by M. Werks, who makes them for CZ, Anschutz, among others. The most appropriate bore guides are made specifically for a particular receiver, such as a CZ 455, for example, or an Anschutz 64 action. I've never used a generic, one-size-fits-all bore guide, but I suspect they are not especially satisfactory for any rifle.
Model specific bore guides are not expensive. For example, the Possum Hollow bore guides offered by Nordic Marksman are under 30 dollars. If you value your firearm and wish to clean it properly, a bore guide and a good one-piece rod are an indispensable investment.