Are you doing this on your own? There are several ways to install inserts. This is something you would normally work out with your contractor. If you are doing them and don't have access to a brake, you'd normally pull the interior stop, back caulk the exterior stop, set the window, foam/seal it, and install the sill trim (expander).

You could flash it if you want. Dont run it past the frame if it is not gonna be covered. 3m and Dupont make good stuff.

Thanks, @Windows on Washington Ltd and @toddinmn. How about Pella smartflash tape? It's butyl based, so it should work.

Thanks!

Thanks again, @toddinmn. For stuff like 3M, I presume that silicone/sealant/caulk can be applied over it and sticks well? Thanks.

What window is coming out? We could be talking about different things here. Are you removing a wood window?

Thanks, @Windows on Washington Ltd. This is a pocket/insert replacement of an 80-year old double-hung window with outside installation. New window is a Marvin Elevate insert. Page 3 of the PDF I posted in my original post (https://www3.marvin.com/WebDoc/Elevate%20Insert%20Double%20Hung%20Installation_19915296.pdf) suggests that Marvin does not call for flashing tape on top of the old sill, just two beads of caulk. I am asking whether, as a matter of precaution, it would be a good idea to put flashing tape on the old sill and then the two beads of caulk. The question is whether 3M flashing tape accepts caulk on top. Thanks.

You do not need flashing tape in this case. If it's an exterior installation, you are going to need to replace the exterior stop with either some "other" stop, whether PVC or wood...or cap the exterior trim. Flashing tape would be redundant in this case and not required. Sill angle, foaming, sealant, and a stop is all that is required. If they are capping the window, the stop is not required.