Hi,
Bellini answer fits my thought.
We have released this website: www.gr1ps.com, totally realized on PS 1.7 platform with a custom theme.
Functionalities we're using are very limited and frontend works pretty well.
On the other side backend has still many issues and we cannot guarantee a full access to the customer, because if he does not behave carefully in some circumstances he may compromise data (i.e. prices in a multistore environment or product taxes).
So the answer is: depends.
If you've been hired for a long term project you may consider 1.7 (since is the "future" version) knowing exactly what its limits are, what workarounds may avoid some issues. Moreover keep always an eye on release notes and development timeline. It may help you to take this difficult decision.
We picked version 1.7 because customer decided not to re-invest for a porting (1.6 > 1.7) to this platform, assuming his risks.
At the moment everything is going ok, but we have to deal with data entry and some configurations (and this is an additional cost for our customer), just because we don't feel confident in giving them a backend access as administrators.