Does impact factor of journal you publish in matter when getting a post doc?
[Note: my experience is from mathematics.  I guess most will still be applicable in other fields, but I don’t have first-hand experience.]

It probably doesn’t matter directly.

For any kind of job application, grant application, etc, you will be assessed on (among other things) the quality and notability of your research.

How exactly people assess that will depend on how close they are to your work.  Specialists in your own subfield will be able to judge your work on its own merits, regardless of where it’s published.  People in related fields will have to judge it indirectly, and may look at (among other things) the prestige of the journals you publish in — but they probably have a better idea of the journals’ reputation than just the impact factor.  The impact factor may matter most when the assessment is by people outside your subject — for instance, some government research agencies or university administrations have official criteria that include impact factors or similar metrics.

For post-docs (at least in my experience in maths), the main people judging your application will be your potential research group leaders, and their immediate colleagues.  So they will be able to judge your work directly — so impact factor probably is not too relevant there.

Long-term jobs and grant applications are much more likely to be assessed, in part, by people not in your own field, or according to fixed criteria.  So for these, impact factor and similar metrics are more likely to matter.  (How much they matter depends a lot on the specific country, institution, and field you are in.)  However, I’ve never heard of the criteria being as crude as “the sum of the impact factors of the journals you published in is close to 10”.