For a good recent discussion on considerations in using overlapping ACS periods, see this post in the ACS Data Users Group discussion forum. In short, overlapping periods can be compared, but they’re harder to interpret correctly, and for small counts, like those in block groups, there’s typically not a strong enough sample for differences between overlapping periods to be meaningful.
Also, for the year 2008 (the first in your study period), the best corresponding 5-year period is 2006-2010, which is centered on 2008.
If you want to compute margins of error (MOEs) for any derived statistics (sums, proportions, etc.) it is very important to use the correct formulas. See Appendix 3 in this Census Bureau handbook for a good explanation with the required formulas for the most common cases.