Continuing the above reply....
'Salvation' or 'Soul Liberation' is nothing but leaving this material world with cycle of Birth and Death and reach that 'Bramham'. So the destination is one, but there are 3 major routes for that same destination. Those routes are called as 'Siddhantas' (Philosophy). They are Advaita, Dwaita and Visishtadvaita. Sri Adi Sankara, Sri Madvachariar and Sri Ramanujachariar were the three main Gurus respectively of those Siddhantas, who revived Hinduism in the medieval period. People can choose any of these Siddhantas according to their tastes.
I'm always interested in exploring and knowing about all these things. Hence I've explored ISKCON too along with others. Their siddhanta is 'Visishtadvaita'. They don't have any separate Sacred Books like some other Spiritual / Yoga movements. They're also following the same Shastras. Since ISKCON is inclined towards 'Sattva Guna', they'll prefer 'Vishnu' and 'Vishnu tattvas' over others. That's what I've explained in the 2nd para.
Hence as per any scriptures (ISKCON scriptures included), there's no such thing as if we worship Shiva, we'll go to hell and all. It's simply few people there who are posting such untrue things. Again as I said in my first para, each and every person will have their own opinion and interest. That's what they're expressing in such sites and blogs. Hence if you have more affinity towards 'Lord Shiva', worship him with full devotion. No need to have any second thought about it. Personally, I like 'Lord Shiva' very much. Hence most of my Profile Pictures would be either 'Lord Shiva' or 'Harihara'.
Ishta Devata worship is for material desires. If you want any materialistic benefits, your Ishta Devata will be more favorable to you than other Devatas. But they're not for 'Salvation'.