I get this question often, so I figured I would post.
IF your ICO token is a true "security" under the Securities Act/Howey, generally speaking, a segregated account/escrow is required for investor funds under Reg D where the offering is made "all or none" or "part or none". Alternatively, if the offering is made on a "best efforts" basis, no segregated account/escrow is required
However, if your ICO token is of a utility/license nature and decidedly not a "security", then the requirements of Reg D would logically not apply. Thus, arguably, no segregated account/escrow requirements
But, dicta in Howey and other cases suggest that funds acquired for purposes of developing a minimally viable product are likely sold as securities (the thought being that utility tokens sold to develop the platform upon which they will be used may be deemed securities)
Also, one should consider the liability issues related to raising less than sufficient capital to reach minimum viability.
I discuss in some detail here:
https://youtu.be/SsBPg1C1lEYAs always, I welcome thoughts/comments. Cheeers