If html has more thant one SSI level apache only cache parent html, for example:

test1.html with that content:
   <!--#include virtual="test2.html"-->
test2.html with that content:
   <!--#include virtual="test3.html"-->
test3.html with that content:
   <!--#include virtual="test4.html"--> 
test4.html with that content:
   test4

In that case if we request test1.html we will get a MISS fromt that page and the second request we will get a HIT, but if we request directly the second time test2.html,test3.html,test4.html we will get a MISS, and I think its better to get a HIT, for example test3.html can have different Expiration time than the other ones, and thats useful to have more dynamic content with less apache load.

I have tested with that configuration mod_cache like this:

CacheSocache memcache:localhost:11211
CacheSocacheMaxSize 10240
CacheQuickHandler Off
AddOutputFilterByType INCLUDES;CACHE;BUFFER text/html

CacheIgnoreCacheControl On
CacheIgnoreQueryString Off
CacheStoreExpired On
CacheIgnoreNoLastMod On

And in VHOST:
<Location />
   BufferSize 1024000
   CacheEnable socache
   SSILastModified On
   SSIETag On
</Location>