The filters in the newer versions of JOSM are great. Check them out if you haven't used them. For instance, if you need to change a tag or add a tag for several items at once.
First create a filter with the tag you want to find and click add to selection. Then select all three check marks. If all went ok, you should see just the items you selected. Now you can highlight all the items you selected easily. You can add tags all at once.
This may add the tag to individual nodes as well. If you want the tags on just the ways, use the filter "type:node whateverthetaghere". Now you can select the nodes easily and delete them all at once.