when you guys make custom taps, where do you store that tap repo/ create the tap? do you keep it outside of your meltano project root? (for example - would you keep it in ".\etl\meltano_project_root" or just ".\etl") and do you install poetry / cookiecutter in your meltano venv?

I would keep your tap repo outside of your meltano projects it is recommended to utilize poetry which will keep the venv separate for you. https://sdk.meltano.com/en/latest/dev_guide.html#building-a-new-tap-or-target

I agree with @BuzzCutNorman 😄 , I've kept it in ./taps / ./targets when I have something specific but I almost always kick myself and need to use the thing elsewhere later

if i will never use the tap outside of bi, is it alright to keep it at the root of my bi repo?

If you want to put it in the bi repo that is your call. Personally I choose to go the separate repo route and have not regretted that decision.