When you first run the program just choose the highest firmware, it's of no consequence.
The method I use is this,
1) - Browse and select the ISO file.
2) - Click "read infos"
3) - Click "Analyse Risk"
4) - Click "Brick Block"
Done
You can repeat steps 2 & 3 just to check if it actually removed the update.
Using "brick block" button that removes all firmware updates. Thats the safest way. It also means you don't need to have preloader installed on your wii, which has been known to brick a few consoles, and saves having to only download scrubbed games.
Just remember to never update your Wii using the console itself, it it tells you there's an update just don't do it.