Clustered indexes - When to use them?

Hi,
Is there any "rule of thumb" on when to (not) use clustered indexes?
What appen to the table/index? (any change on the physical organisation?)
I've seen speed improvement on some queries but I'm not sure if I must use
them or not...

My rows are imported in batch of 100 (once the main script has collected
them, this takes between 1 and 30min), then another script vacuums the
table and aggregate the last imported rows, if I add a column with the
commit timestamp and cluster on it, will I gain some perfs or not?

Thanks,
--
MaXX