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If you really want to use a wormhole to travel in time, you have to find a way to survive the journey through it first, because of its unpredicular nature.

 

Yes, wormholes of even microscopic size are incredibly unstable. However, the effect I mentioned prevents them from closing at random by being stronger than the curve of spacetime trying to close it. I think the worst part about traveling in a wormhole would be touching its edges. I know if you touch a purely 2D edge, you can be cut pretty badly due to it sliding between your molecules with no resistance. A 4D edge may increase the complexity of your geometry in a way much different from a cut, to a point where you simply can't exist as a 3D object anymore, but I'm basing that on the assertion of spacetime having a non-Euclidean geometry like what we normally observe all around us. Honestly, it has no other base and is probably not close to what could happen. The absolute worst thing I know would happen is if the portal becomes unstable during your transit, and closes on any part of you. Depending on how (or in this case, where) you cross thresholds, this could be particularly disastrous. Imagine suddenly being a galaxy away from your own genitalia, and you're in space. Now your spacesuit has no structural integrity and all of the air you were breathing is pulled into the vacuum. You have to exhale to prevent the oxygen in your lungs from destroying them as it expands, and you have less than a minute to get back inside and tend to your crotch-hole.

 

Theory is fun; reality not so fun.