Is it secure too use adobe flash player on imac 27" with mountain lion 10.8.3?

It is safe to use Adobe Flash Player.

But keep it updated always. Adobe releases security updates frequently.

System Preferences > Other/ Flash Player > Advanced > Updates > Check Now

Best.

If a security bug is found and becomes known Apple will disable Flash on your system pretty quickly and block it until an updated 'secure' version is released. This isn't a rare occurrence, given Flash's history.

Make sure that you only install Adobe Flash that is downloaded

directly from Adobe. This is the only way to truly guarantee that

it is an Adobe copy and not a malware or trojan carrying copy.

Also, only update either from the Adobe Flash in System Preferences

or download directly from Adobe. If ANY website tells you to download

an update, always check in Sytem Preferences or directly from Adobe

to determine if that is the case. Too many people blindly click yes when

prompted and many end up with various malware on their sytems as

the malicious people make it look very official looking.

As a general rule when using the internet, if you didn't ask for it,

do not accept it!