First, go with a vanilla cfw unless you need and use the features of the others. Vanilla being as close to stock as possible.
Second, only install homebrew such as backupmanagesr, psnpatch, etc that are stealth versions. This means they reside in an official ps3 hdd directory that a old game or official product used.
Third, never launch any homebrew or have any non stealth installed when going near the internet.
Fourth, after loading a game with your stealth manager (or delete it after loading game to xmb) delete any homebrew that isn't stealth like showtime for eg and run psnpatch or equivalent to both delete history and disable cfw.
Fifth, never connect with a lower firmware than used before.
Sixth, connect to the internet, log in and run the game from xmb which was previously loaded by the manager.
You will get console banned not just account which is reversible if you can buy/retrieve another set of id's.
This may be a necro reply but okay
Since you've listed the precautions, I'll list step by step for PSNPatch
1) Get PSNPatch, it disables CFW syscalls and all that (It renders CFW unusable including homebrews and backup managers)
2) Do not be on PSN at the moment (You shouldn't be when you're running homebrew)
3) Run backups manager and mount the game you want to play online
4) Run PSNPatch and disable CFW
5) Connect to Internet
6) Run the game from XMB
7) Sign into PSN while in-game (You can do this before running game from XMB)
Doing this disables it but only after you mounted the game.
To reset CFW, reboot the PS3
I'm thinking to get a soft modded PS3, I don't want to play online and go the PS store.
But I only want to update my trophy list once in a year.
You could do the above precautions if you want to, it's a must if you do online gaming though
If you're only going to sync trophies annually, you can just do it since you're on there for such a short span of time, not very likely of ban but risk is still there.
Might be somewhat suspicious if a crapton of game suddenly appear though (Imagine a person purchasing 40 games in a year)