safe to use an unshielded jack for 10Mbps Ethernet?
Unless the application is in a very noisy environment shielding is not required, or really even recommended.

Shielding may be needed in very noisy environments such as some industrial settings but it comes with its own set of problems.  

Special attention needs to be paid to ensure that the shield is properly grounded, preferably only on one end of the cable.  If both ends are grounded then both systems need to be able to deal with the ground potential offset that will almost certainly exist.  In more complicated network topologies this can be a serious issue.

In short, if you don't REALLY need shielding, don't use it.