Share user information across multiple sub-domains
I am trying to share user information across multiple sub-domains and came across this very good URL which explains how to achieve that using db_prefix.
I am using V6.x and the way I started was that, I shared only users, users_roles and sessions tables across the domains, since I want every thing else to be different across sub domains.
These are the tables that I see drupal creates with a default installation of V6.x for each domain/sub-domain.
access, actions, actions_aid, authmap, batch, blocks, blocks_roles, boxes, cache, cache_block, cache_filter, cache_form, cache_menu, cache_page, cache_update, comments, files, filters, filter_formats, flood, history, menu_custom, menu_links, menu_router, node, node_access, node_comment_statistics, node_counter, node_revisions, node_type, permission, role, sessions, system, term_data, term_hierarchy, term_node, term_relation, term_synonym, url_alias, users, users_roles, variable, vocabulary, vocabulary_node_types, watchdog
what other tables from the above list can I safely share across domains, and still ensure that there are conflicts between the way various modules use, create data in to these tables.
I also want to know if this is a best practice, since I have plans to add around 20 sub domains in a long run and each sub domain will add around 40 tables to the existing database. Is that good to run the whole list of sub domains all from the same backend database or is there a way a better way to share user information especially on such a big domain containing various sub domains ?