I have seen those guides advertised and thought about getting one, but never did.
I can see where it has some advantages (if it works), especially in keeping the slot from being "belled" (oversize where it enters/leaves the barrel and tight in the middle).
For a person who has not done a lot of filing, it is a challenge to keep the file level and perpendicular to the barrel at all times. I think the guide would help in this.
To some of the people who have built a lot of guns, filing becomes second nature and they can zip them in without difficulty but this is an acquired talent.
With the underlugs, it isn't a real big deal because if they are tapered/belled, no one is going to see them anyway.
For the sights, especailly the front sight which is standing proud for all to see, a screwed up dovetail can be painfully obvious.
This is why on my early guns (and a few of my later ones) I just paid the $5/slot to have Dick Greenside (Pecatonica) cut the sight dovetails for me. (I also tried a local gunsmith who charged me $20 per dovetail once.)
If your barrel doesn't have the dovetails cut, I would suggest that you try it on the underlugs. If you don't have a problem your home free. If you have a problem, you might then want to consider buying the jig before you attack the sight dovetails.
By the way, for the underlugs, I like the stapled type. All they require is using the double punch tool to mark the location, drilling 2 holes, tapping them in place and staking them with the special hollow stakeing tool.
(Yes, I know, Not PC!).