How should a researcher manage the fact that Impact factors change over time?
Related to your last comment:


  boards that considered only the papers published in journal above a certain impact factor when assessing the quality of a candidate


I think that such borders are not doing well. As you said, the IF (Impact Factor) varies in time (regardless if we consider it as a relevant measure).

According to me, there are 2 points of view by which the IF which should be considered in your question:


The point that you tried (and succeeded) to publish a paper in a journal with high IF. What  would that fact say to me (if I would be in a committee)? That you were confident about your results and you trusted your work to be published in a good journal. (BUT(!) it does not meat that the journal or the paper is good, it just reflects you, the candidate). In this case relevant is the IF by the time your article was accepted.
The point that the IF of the journal you published rises in the next 2-3 years after the time your paper was accepted. Why? Lets take a look how the IF is calculated:

A = the number of times articles published in 2006 and 2007 were cited by indexed journals during 2008.

B = the total number of "citable items" published by that journal in 2006 and 2007. 

2008 impact factor = A/B. 


However, can we say that articles published in 2006 and 2007 will be cited in 2008? Just very small amount of them. Someone reads your article (month 1), then does the part of research related to your article (2-3 months?), approved by co-authors (another month) and goes through the acceptance procedure (12-14 months). It takes about a year and a half to print the citation of your article. So, if the editors decide that they will accept just good articles the resulting rising of IF will be visible in 2-3 years in the future. So if I would be a committee member I would like to know how was the IF 2-3 years after the article of the candidate was printed. That can tell me that by the time the article was printed, also a considerable amount of good articles were printed as well. But keep in mind that IF consider citation just within 2 years back, and that is just too short. I think it should be 3 years at least.

So, to answer your questions:

add 1.: YES, for the next 2-3 years as it is effectiveness time of IF. 

add 2.: YES, because it reflect the actual state of the author by the time he decided to publish.



But please, all this can help as a indicator but is should be taking into account with caution. I should also say that IF is very tricky and it can be easily misinterpreted (self-citation, how many review articles the Journal publishes,...). If you want to take a look at other evaluating tools, you can visit:

Eigenfactor or
Journal Ranking

and:
What really matters is how much your article is cited(!)