I just replaced my wheels on my 94 Yota 4x4. I couldn't use the stock hub covers, do I really need these? I replaced the wheels with the 15x8 American Racing outlaws (I think that's the name). They look 100% better than the stock steel wheels!
By hub covers, I am assuming by my lack of knowledge that these are a tin-can metal cylinder that fit over the hub...dressing up the the wheels. I had a ten year old set on my Toy 4x4 when I bought it, and found that the steel hub covers rusted to the hub! It was not so bad that it was impossible to get off, but I lived in SoCal where it never snowed (no salted roads). I ended up throwing them in the trash and have never looked back.
edit: After re-reading the post, I see that you are talking of the STOCK part. The hub covers I had were cheaply plated chrome aftermarket pieces. They are not needed for manual locking hubs.