Metal cassette's and bias settings?

Could someone tell me why bias controls don't have much affect on Metal type IV cassette's,and is it true that TDK tapes don't last as long or hold onto there music as long as Maxell cassette's?And are TDK's more prone to dropout's and loss of high frequency than are TDK over a shorter period of time ?This is just what i have been told before and i don't know,I was told to never put anything that i don't want to lose on a TDK cassette because after about 5 years or so there start to exibit the above problems..Is any of this true or is it just "hogwash"? Thanks for any help,Keith..