Should I upgrade this wallet every time if I'm just holding some coins?
No reason to upgrade if you don't plan too stake or move coins. Just upgrade the next time you need too move coins.
wrong. It is a mandatory upgrade

it's a mandatory update but as long as you have your private key(s) your are fine. If you don't plan to send coins and you're not staking them you don't have to update anything. Every wallet works this way.
Anyway, for all the others:
Update your wallet guys!Sooner rather than later he will be sending coins anyways so why not do it now lol. Besides it only takes 2 minutes, you only download the ZIP file. Replace the old wallet with the new one and you are good ( if you have your wallet.dat in %appdata% it will sync automatically )
His question was whether he should update if he plans to hold only, the answer is no. Not everyone is able to stay on date with all the update we have.
You are making it harder than it is lol.
Devs: This is a mandatory upgrade.
JJack@: ok
JJack@: Thanks, it took 2 minutes and my coins are now safer!

You seem to lack basic understanding of how DeepOnion (bitcoin) works. There is a lot of good arguments why you should upgrade. That the coins are safer is simply not true. Your coins are much safer without an upgraded wallet installed on your computer.
If you plan to hodl your coins you should encrypt your wallet.dat, and delete the wallet from your online computer. (And only install when you plan too send the coins. If he wants to send the coins 2 years from now, there will problably be a lot of hard forks before that. It is a complete waiste of time installing every upgrade and it is risky storing private keys on an online computer.
No offence but while your argument is true that a wallet that is offline is safest, your argument is also flawed. DeepOnion is a PoS coin so needs to be online at some point to claim the mining reward, a wallet stored offline for years on end will miss out. Secondly, it is my humble opinion, which I know many security experts support, that there is no such thing as a perfect piece of code especially as it becomes more than a few 100 lines. The fact that you need to have your wallet online for reasons other than sending means it is very important any security risks caused by human error in the code need to be fixed if found, rather than left for someone to exploit, this exploit could lead to someone hacking your wallet and stealing your coins, so yes, there is a great possibility upgrading your wallet will make your coins safer.
Unless we know what security risks were patched, it is difficult to say that this fix did not make our coins safer.
You are right, 10% of PoS rewards (which will change to 5% in July and later to 1%) is not a small percentage for holders. Also, updated wallet which is online is a ToR node (not an exit node) which makes the whole network stronger.
Most people should definitely upgrade and stake their coins. But if someone just wants to hodl, like bbv4pawn, it is not necessary. And that was what he asked about.

Big holders will upgrade and stake their coins. If some smaller holders don't it will not reduce the security of the coin in any signifcant way.
Yes agreed, if you do not plan to stake or spend your coins there is no great reason to have a wallet with all of your ONION online and if you aren't using the wallet regularly the benefit to having the latest wallet is diminished.