indexing

Hi all, More of a database question here, my apologies: I've been doing some reading on indexes but I thought I should get an experienced persons input. My question is about "Over Indexing". I have a MySQL MyISAM table that stores inventory transactions, in, out etc. I store numeric fields that are primary indexes in other tables, 2 datetime fields and a qty field, Paul DuBois' MySQL book says that anything used in a where clause or join clause is a candidate for an index. It turns out that 7 out of eight fields fit this description,1 primary key, 4 smallints, and 2 are datetime fields. Should I use all these indexes or is this overkill? Thanks for any input Steve.