I agree with with other posters about 'We decided it was a good idea.' In English, when the first verb is in the past tense, it is usual to put a second verb in that past tense as well. 'He decided it is a good idea' does make sense when the idea is one that exists in the present, but 'He decided it was a good idea' is an equally correct translation of exactly the same idea in English - in the past, he decided that it was a good idea. Compare with 'She decided she was going' and 'She decided she is going'. I also can see how, if you translate the first verb in the present perfect, 'We have decided', then 'it is a good idea' follows as it's also in the present tense. But because the first part of the sentence can also be translated 'We decided', then 'it was a good idea' should be equally accepted.
Okay okay. The translation "We decided it was a good idea" makes perfect sense to English speakers who have no problem seeing that sentence as present tense. Hebrew speakers would insist on, "It is a good idea," as the correct translation because, let's face it. They don't know English. They only think they do.