Yes treat your entire airframe. You'll get some contrary opinions based on people who've never discovered corrosion on their aircraft because of various reasons or never had a problem because of their mastery and selection of the perfect conversion coating/primer/paint system, etc. I owned a Mooney that had several areas where access was painful and not required at annual. The spar caps were also 7075T6 which have a higher tendency to suffer IG corrosion when exposed to catalyst (moisture). It's not unusual for one of those to get "totaled" because of wing corrosion. As there's no insurable event, the owner is often left with an assembly of salvage parts at that point.
There are people who you can pay to do it (they already have the equipment); maintenance shops, hangar neighbors, etc. You can buy equipment from the preservative suppliers or there is a plethora of fogging equipment out there or you can buy from the preservative suppliers. The biggest "dedicated" equipment benefit is probably the very long wands that will reach >1/2 way down the wing.
Which preservative? There's no clear best, IMO. See this thread ~ post 5 for a summary from the way back machine.
https://vansairforce.net/community/showthread.php?t=197941
Want the best creeping/easiest atomizing? Probably ACF50 (witnessed it lift/remove bluing from a firearm). Downside is it will get through the seams/joints and some rivets to your paint. Atomizes so well you can see the fog coming out of access panels, other end of the wing when applying.
Want the one that best met the applicable mil-spec? CorrosionX. Lays down thicker and would probably protect better against the unusual (urine, battery electrolyte, soft drink spill, etc) Downside = it weighs a little more. Seems to collect dirt better. Didn't atomize quite as well or creep as well as the above.
Want to keep the mess down and save your paint job? Dinitrol AV-8 or equivalent.
All of these are very good products though someone will argue that their selection/knowledge is better than anyone else's. Are you already fighting corrosion issues? Where is the aircraft in it's service life? Is the paint still brag worthy? Hangared/outside? Environment? Got access to real fogging equipment? Answer these questions and proceed. Any is far superior than nothing. It's very cheap insurance.
My lord, you might lose a fraction of a knot from the weight; of course, you could go lite on breakfast on flying days and get it back. The main thing is to protect your safety and investment. My opinion of course, for what it's worth.
Stay safe.