I use Zero regularly. I’m on the maxium APR – 35%, so tend to use Clarity more (plus Clarity lets me have £500/day).
But despite that, I would never pre-load. It’s against the T+Cs and could trigger a block.
Even on my interest rate, if I take out £100 and pay back a week later, the interest will only be 60p. In practice I pay back in a day or so, so interest is tiny.
I did once accidentally pre-load Zero by a few hundred quid. I simply doubled up on paying the statement balance. The card wasn’t blocked and eventually the positive balance was used up with further ATM withdrawals.
So, the advice on here will always be above – don’t preload, it’s against T+Cs and your card might be blocked. But there are few stories (if any) of this happening. Still – not worth the risk in my view.