Yes, I wouldn't even think about installing a motherboard using only two stand-offs. You need to use all the ones available (which line up with the holes in your mobo of course) otherwise you can expect to have shorts etc against the back of the case. What case do you have? It's pretty poor that a case would only come with two standoffs. There may be some in the motherboard box if you're lucky though it's a shot in the dark.
Luckily, standoffs aren't all that expensive to buy and you can grab a pack of 25 etc. so you always have a few spare for the future. I'd be dropping an e-mail to the case manufacturer though to express your disappointment.