It is a bad idea if you have stored the database
in a network share (in "my Documents' if you are
using roaming profiles). You can loose the file
if you run out of network space: you can corrupt
the file if you have network problems, and it will
be really slow anyway - so slow that you may loose
or corrupt the file by turning off your PC during
compaction. Compact on close can be useful if you
have a lot of transient data, causing bloating
of the database, and you have a separate backup
of the database anyway.
If you have a local database (with backups of course!)
then you may just try it and see if you like it.
(david)