flashing roms in a custom recovery only writes to the boot and system partitions,so there is no danger of accidentally overwriting something important,as long as your flashing roms intended for that device.
"good" flashes shouldnt hurt,no matter how many you do. bad falshes (from an incomplete download,for example)
could hurt memory blocks,wich can have an effect if you get enuff bad ones for there to no longer be room for a rom
that said, i would repeat what the others have- dont worry about wearing out your flash memory,just make sure you flash only complete,working downloads by comparing md5 sums and only using roms from reliable sources.
if youre s off,parts of the phone that do not live in flash memory(like the radio) are destroyed by bad flashes,possibly unrecoverably,so use extreme caution what you flash via the bootloader,i personally do not like to mess with hboot,radios,and other firmware any more than neccessary