When is it okay to use accidentals.

[egoopus]

I know you can use accidentals when using second dominants and when modulation but what other situations allow you to use accidentals.

[quantum]

When you need a pitch that is not referenced in the key signature. 

For a period of time when brass instruments did not have valves, key signatures were not used in scores.  Everything was written with accidentals.  Take a look a some Beethoven symphonies for examples. 

Or is there something specific you were looking for?  Can you elaborate on your question.