Effects insert wiring.

[Kesh]

I'm putting a TRS stereo insert jack on the amp I'm building.

I wondered, what is the convention for which, out of tip and ring, is FX send, and which is return?

[FiveseveN]

A quick Google search brings up both options in fairly equal abundance. But Wikipedia says

Most modern entry level and medium format mixers use a single TRS phone jack for both Send and Return. This dual-purpose insert jack only has three conductors, and balanced lines need at least two conductors. Because two lines share the same three-conductor insert jack, its architecture is necessarily unbalanced, with the two circuits sharing a common ground. Of the mixers using this kind of dual-purpose insert jack, most are designed with Tip Send, Ring Return, though many can still be found with Ring Send, Tip Return. A very few mixers have both architectures present on the same mixer; Tip Send for input channels and Tip Return for mix groups.

It's really no big deal, as these inserts are always used with Y cables and switching from one standard to the other just means swapping the TS jacks.
The configuration should be noted on the device just to be sure, like here.