Commas in "if" sentences

I was under the impression that we can't use a comma before 'if' for the following sentences:

- Please let me know if you need any further information.

- We will get wet if it rains.

When using Cambridge Advanced Learner's Dictionary though, the following sentences were used when looking up 'would' and 'could'.

- I wouldn't worry about it, if I were you.

- We could go for a drink after work tomorrow, if you like.

Why is a comma used before 'if' in the would/could sentences, but not in the first two sentences?

Thanks!

Sooris wrote:

Rule: If the dependent clause comes first, you should use  a comma. But in your example sentences the first clause appears incomplete. May be that is the reason a comma is used there.

Thank you Sooris, but I am still confused.

I thought the first clause of these sentences are complete, independant clauses, aren't they?

- I wouldn't worry about it.

- We could go for a drink after work tomorrow.

So still, why is a comma used at the end of these sentences?

Thanks!