When applying for postdoc funding, what are the consequences of backing out if I can get another position where funding is already secured? When looking for a post-doctoral fellowship, I have, in a nutshell, two alternatives:


Apply to open positions, where money is already secured (at least for the beginning)
Apply together with a potential supervisor for funding agencies, such as NASA NPP, UCAR VSP, NRC, or one of the more generic funding agencies.


With option (2), I'd need to first convince a potential supervisor, then write a funding proposal to convince a funding agency.  From a candidate's point of view, (1) is easier.  On the other hand, there are a lot more options in (2) to try, than in (1).  My question relates to option (2).

Two relevant calls for me close on 1 November and 10 January.  Results are announced in February and March.  I could apply to both, with different projects and different supervisors.  While waiting for results, other opportunities, where funding is already secured (such as point (1)) may come along, and I'd apply to those too.

Is it unethical, unprofessional, or otherwise unwise, to apply to different programmes, with different supervisors, and still apply to immediately available positions while awaiting the result?  Suppose that I find a postdoc elsewhere before even knowing the result of either funding application; can I then simply back out of the still-open funding applications without consequences, or should I expect this to cause severe annoyment with the host scientist I'd be applying with?  After all, he or she would have invested a non-neligible amount of time in the proposal.  I find it different then simply applying for different jobs, because time invested is much less.  On the other hand, I don't want to have to wait until March, before hearing a negative result and having to start all over again.

I'd like to hear from senior scientists how they would look upon a fresh PhD who they'd spend time writing a post-doc proposal with, but who'd then back out for another position before the result is even announced.
The key is to be open and honest. To answer the question lets define some terms. Let Prof X be a potential post doc supervisor who already has funding and Prof Y be a a potential post doc supervisor without any funding.

Prior to working on a funding application to Prof Y you need to discuss whether he expects you to put all your eggs in one basket or if he is okay with you applying for different jobs. There may be applications that Prof Y will not make with you or it might change the application strategy. For example, Prof Y may not list you as a named post doc so as not to cause problems with the funder if you do not take the position. The key is to be honest and up front.   

If you have a application in with Prof Y and Prof X offers you a job, then you have problems. If it is close to the decision deadline for the funding with Prof Y, you may want to wait before starting with Prof X. Again you need to talk with Prof X. Maybe Prof X X needs you to start right away but instead of a 12 month commitment he only needs a 6 month commitment and maybe you can delay the start of the funding with Prof Y, if you get it. 

In general, most people understands that a guaranteed job is important and will try and be flexible. That said if you have funding, you need to deliver the work you promised, and you don't want to apply for funding you cannot use.