if you pc is clean being connected or not will not make any difference,[1] if you knwo that your pc is infected(usually something wrong happen in this case, like cpu usage too high, strange filename ecc...) then it is better to move your wallet in another storage(offline is better)
my point is that you can have a online safe computer, in my case i have a wallet that it is always online with some btc, no one has stole those bitcoin from my desktop ever, they are still here[2]
[1]Yes it makes a difference. If your PC is disconnected a hacker can't gain access to it. While it is connected, he can. There are infections that can't be detected by anything, not by the average person nor software because of its complexity. However, a security researcher (or a group) might.
I'm going to assume that you haven't heard about The Mask (a.k.a. Careto); you should look into it. Luckily AV's can detect it now, however there are probably a lot of these out there (e.g. finfisher).
[2] If nobody has stolen your coins, that doesn't mean that they won't. Depending on your balance you might not be a likely target. We've seen a lot of people (over time) ask for help on the forums because their wallets were compromised.
if my balance is still here(about 5 btc), this only mean that my pc is safe, and not infected, it's impossible that i have a wallet stealer, and still i have all my coins here, it does not make sense at all...
same thing for a keylogger or any other virus, none of the forum that i usually visit are compromised, my bank acount is also safe, this can only mean that my pc is 100% secure
just don't download trash(only signed file) and you're safe even online, hackers cannot connect to your pc magically, they need rootkit/rat ecc...so it is actually you that allow him to connect by installing those garbage
ah great now, plenty of newbie scammer are sending me pm , they want to try to scam those 5 btc i presume