I think Snape can be trusted.
1. Dumbledore has proved to be an excellent judge of character.

2. Snape came back to the Ministry of Magic before the downfall of Lord Voldemort and gave crucial information.

3. He tried to save Harry's life on numerous occasions.

4. His own life was saved by James, since he's dead, Harry is the one Snape has been paying his debt too, he won't see him dead.

5. He didn't return to Voldemort the night of his return, I believe he was the one Voldemort said would be killed for treachery.

6. He wasn't (or wouldn't) able to give Umbrigede the veritaserum to question Harry (and he hates Harry!).

7. Snape interpeted the plea from Harry in book 5 about Padfoot being in trouble and acted upon it.

Need I say more? Snape isn't a perfect man, but he has his priorities straight.