there is nothing wrong with undervolting
theoretically it would do exact opposite of overvolting i would think
i had my 1ghz tbird running at 1.4v for over 5-6 months and only thing i noticed was cooler temps 🙂
sounds like its stable enough, but if you want super stable
download 3dmark2001
start up prime 95 torture test and loop 3dmark about 50-60 times (it will take quite a while for it to end, maybe overnight or something would be nice)
if it withstands that, then id say you are good to go