First of all, safe-deposit boxes are not themselves assets -- it is the items inside the box that are assets. If the safe-deposit box has joint access by both spouses, then you should at least inventory the contents at the time of (or immediately prior to) the initiation of a divorce. It is not a bad idea to take photographs and to take notes on what is in the box so that you have proof. You should date the notes and make them as detailed as possible. It is not a bad idea to get a copy of the log sheet from the bank showing the date when you did the inventory. It is also very important to have a neutral third party go with you and act as a witness. The third party neutral can then initial the inventory and verify the contents. If you cannot find such a person (preferably not a relative or friend who could be accused of being "biased"), someone like a bank employee might even be a good person to use for this. By doing this, you will be able to prove the existence of items in the event of a dispute. If the items are very valuable, it might not be a bad idea to take an independent, third party appraiser with you so that you will have someone who can testify as to value in case the items "disappear." You can usually find someone like a local auctioneer who can perform these types of services.
As long as there is no temporary restraining order or temporary injunction in place, you could potentially take the items and put them into your own safe-deposit box or other safe location so as to limit access by your spouse. If the items are your separate property, (i.e. - the items were purchased prior to marriage, or obtained by gift or inheritance), these actions might not be view by the Judge to be unreasonable. However, if there is a temporary restraining order or a temporary injunction in place (or as we have in Austin / Travis County, Texas a Standing Temporary Order) then you should not and cannot take the items (hide or secrete them) without Court approval. You should keep in mind; however, that if you start taking and hiding assets of the marriage, your spouse might start doing the same and this would not be a good thing.