I think this would mostly come down to constraints of the "IPO" itself, such as how escrow would be handled etc. Ideally you should offer as broad a base of denomination as possible while still remaining convenient to everyone. ("I'll send you some apples for zencoins." is probably not logistically viable, but "btc, usd, ltc" should be trivial enough.)
I'm not sure I understood you. If you meant trusting someone else to keep on the BTC of the sale, it's problematic. If it's regarding the Zencoins themselves, they won't come into existence until the network is ready, and at that moment they'll be distributed to their owners.
Add to that the fact that many constrains are arised from legal point of view. Zennet will function as a regular company, offering to buy its products in presale (in ridiculus prices of course), while obligating to supply those products.
Of course, no one (including me nor the company) have any kind of control or income from Zennet after releasing it.