many switches do not support mdix, so do not assume that every switch port of every switch does.

It does have to do with the way the Tx/Rx connections are wired though.

The transmit on one side should be connected to the receive on another, and the other way around.

Some hubs have an 'uplink' port, to which you can attach a straight through cable instead of needing a cross over cable. In this case, the uplink port has switched the Tx/Rx wires.

so when we plug a pc into a switch, which device has the tx and rx switched by default, is it the switch or pc ?

Your question doesn't really have a straight answer because it's all about perspective.

The important thing is to remember that the transmit on one side goes into the receive of the other. For a traditional switch to switch (ie not including auto mdix), this means using a cross over cable. When connecting a PC to a switch (or router to a switch), you use a straight through cable. When connecting 'like' devices (eg pc to pc, switch to switch), you traditionally had to use a x-over cable - again, excluding features such as auto mdix, and uplink ports.

The cross over port would normally be on a hub/switch.

This Wiki entry might help explain further: http://en.wikipedia.org/wiki/Medium_dependent_interface