After buildindex is run, when the index is propagated to the slave solr servers, the spellcheck index is not propagated along with the index. This is a limitation in solr (up to V3.5 at least).
With this limitation, your autosuggest terms won't be up-to-date on your slave servers. A change was made in Feature Pack 5, to have the slave servers build the spellcheck index on a commit. In the solrconfig.xml:
<str name="buildOnCommit">true</str> <------ FEP5 Change
If you are building the index infrequently, during periods of low activity on your site, this change should be fine. If, however, you are running frequent delta-updates all throughout the day, you might notice high CPU on your solr slave servers.
Depending on the severity of the high CPU, you might want to consider reverting the 'buildOnCommit' parameter back to false. If you do set this parameter to false, you can trigger a build of the spellcheck index for a specific index by using the following command:
http://hostname:3737/solr/MC_<Master Catalog ID>_CatalogEntry_<locale>/select?q=coffee&spellcheck=true&spellcheck.collate=true&spellcheck.build=true
The Spellcheck index generally doesn't need to be updated as often, unless your delta builds are inserting new catentries. Consider setting up a nightly job sending this spellcheck buildindex command through curls to each of your slave solr servers. This buildindex command will only route to one of your slave servers in the cluster, so it's better to hit each slave solr server directly with the command to update the spellcheck index.