From what I understand, a semi brick can be caused by agreeing to an update that does not correspond to the system you are running. For you, it means agreeing to the update a PAL game may ask for when played on your NTSC Wii, for me in Europe it would be agreeing to an update a NTSC game may ask for on my PAL Wii.
What you are doing (playing only NTSC games on your NTSC Wii) is the safest. Most games come in their NTSC first anyway. If you want to be on the safest side (or if you are paranoiac as I am ;-)), run the WiiBrickBlocker and patch your iso before you burn it.
If I am wrong, can one of the expert on the board jump in and correct me?