You can use it with or without, personally I tho k the rubber caps are more for a persons peace of mind than actual function. Just be sure it is fitted properly as that's where the function comes into play. A mis-fitted prong is really useless and requires a much stronger (and unnecessary) correction to obtain the same results. Leerburg has a nice article to fit a prong.
And of course, prongs are training tools and should be eventually weaned off as well as never left on while unattended or on a flexi/tie out.
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