The primary problem with SNMPv1 and v2 is that passwords are sent in clear text, which means that someone sniffing your network could easily get ahold of those passwords and use them to gain control of your routers/switches/servers.
So, if you are going to use SNMP, use v3, if the device you are monitoring does not support v3 then use v2 and only use a "read only" password. Disable writing capability (if your device supports it).
And, whatever you do: do not set your read only password to public and your read/write password to private. If they are already set that way change them.