Gooey tape rollers - how widespread is this problem ?

Hi I've got a bunch of old equipment that I'm trying to cull the backup tapes for, namely a MicroVAX 2000 with a TK50 tape drive, and a few QIC drives off various PC, Sun and HP systems. The boxes and boxes of tapes need to be culled, and transferring stuff to CD-R / DVD-R, or simply ditching tapes which are redundant is what I'm looking at doing. I've noticed a few recent postings about tape rollers turning to mush if they haven't been used for a while. How widespread is this problem ? Does it affect all brands of tape drives, or are certain tape technologies worse off. I can never recall having any problems with our TK50 drive from the VAX - it was always reliable, but it's been stored for a few years now. As for the QIC drives, they were "inherited" and I've never really used them, so their history is unknown. How old do the tape drives have to be before this problem appears ? It is related purely to age, or does usage play a factor ? TIA Jason