Are lockwires still preferred over self-locking nuts and other ways of bolted joints` safety?
Difficulties and costs of maintaining the lock-wiring are obvious. My intuition says it only has to be used as a last resort when no other way is available. Nevertheless any aviation junk yard says my intuition is wrong. Lock-wires are everywhere in the old gas turbine engines. My question is:
  
  Do they now use other methods of bolted joints` safety in gas turbine engines (jam nuts and locking washers in particular)?


Well, maintaining safety lockwire isn't as hard as you think it is: A pair of safety wire pliers will last you a lifetime, and they do about 80% of the work for you (all the maintenance monkey needs to do is cut an appropriate length of wire, hook up the pliers, and twist, making sure the wire is positioned so as to always tighten the fastener).
It is easy to inspect, repairing is simple (just replace the damaged wire), and cost for a spool of wire is minimal compared to using a new self-locking nut every time something needs to be taken apart (safety wire earns its keep for temporary assembly too: You can wire the assembly when you're done, rather than wearing the lock material on a self-locking nut).

Other methods of securing fasteners are used on aircraft (in engines and elsewhere), including cotter pins and self-locking nuts with nylon inserts to prevent them from slipping. The cotter pin is easy to verify (like safety wire), and is widely used.  Self locking nuts on the other hand can slip without much visual indication (torque seal or similar products are often used to mark the nut and fastener to give a visual indication of slip).  

A disadvantage to self locking nuts is that they must be replaced any time the fastener is removed (reused nuts will have a lower locking resistance, and may fail to hold), and using them introduces a potential for human error in maintenance operations that does not exist with safety wire, which must be cut to remove it. (There are even folks who try to reuse cotter pins, though normally they are cut to remove them and thus cannot be reused.)