I think you do. The tool is quite cheap though If you look at the chainring bolt, the 'female' side is slotted, and the chainring tool is just designed to hold the slotted side of the bolt in position whilst you screw in the hex head. I don't know if you're allowed to post link but Spa call it a SHIMANO-Chainwheel-Peg-Spanner and it's a fiver. You could probably do the job of holding the bolt still with a screwdriver, but the proper tool has a 'tongue' which pushes into the back side of the bolt and stops it slipping out of the slot. Blimey that's a lot of inuendo for one post.