Getting the next sequence value safely

Hi,

I have a situation where I want to insert a record into a table
which will give the record a unique id via a sequence.  Is there
an easy and safe way to get this id number either right before
or right after the insertion?  My best idea is to select last_value
from the sequence and then assume that the record will get
last_value + 1 when it is inserted (the insertion would happen
right after the select).  Is this a safe assumption?
What happens if I wrap these operations in a transaction?  Is
there a standard way of doing this?

Thanks,
--Jeremy