Is the Guild Tribute Chest a safe storage

Solution 1:

No, it's not safe.
You may be lucky and it'll remain untouched for a while, but you risk losing all its contents anytime.

See here: Where can I safely store my stuff?

Solution 2:

The answer is absolutely not! I learned this the hard way. I completed the main storyline (dragonborn quest line) and therefore collected the two or so dragon priest masks you get through that quest line. I stored them, along with several other things in the guild master's chest where I had them for yonks. One day, when I got the latest official game update for the ps3, which fixed certain bugs, I found out that all the items in the guild masters chest were gone. I can't get them back.