Do you always need to replace rotors when you replace pads? I'm talking specifically about my 2002 Volvo S60, front brakes.  Is replacing the rotors done each time, or is there a rule that determines if they need replacement or not?

As long as the rotors can be machined and stay within tolerance there's no need to be replaced.  The minimum thickness of the rotors should be cast into them.

What if I have no access to machine them? Is it ok to just slap the new pads on without machining the rotor as long as it isn't terribly grooved and in thick enough?

There's no reason not to machine the rotors and simply install new pads...but you might get some shimmy when pressing the brake pedal.  If you get no shimmy you're gold...if you do get shimmy it depends on how bad and how long you can live with it.

It's usually best to machine the rotors, but not always necessary. Really depends on the condition of the rotors....and how bad you really need the car to stop. If there are no gouges, or deep grooves cut into the rotors, replacing the pads is ok. If the old pads were down to metal/metal contact with the rotors, the rotors really need to be machined, or replaced. You can put new pads on them, but they won't last very long.

Our other car, a daily driven VOLVO sx90 came due for brakes and the dealer said that they donot cut rotors so they only replace them , Mine were worn but not gouged nor grooved . I think I got taken . As said in prior post, there is a chance of shimmy . Try it ,you may be lucky Big Al

My daily drivers go for 250,000 miles plus.  I used to have the rotors machined with every pad change.  Way back when it was just a $10 charge to check and machine.  As of 10 or 12 yrs ago I couldn't find anyone who would machine them.  Now I just change the rotors every other pad change.  With the cheap prices on rotors, at least on the cars I have owned, it seems like the cost of getting them turned would probably exceed the cost of the rotor.

It was just at the Volvo dealer who wanted to put pad on and replace the rotors.

Yes grease the slide bolts, all the salt and grime does hold them up some times and with thickness of the new pads they will be in a different position.

Most(but not all) vehicles since the mid '90s have throwaway rotors...meaning they are not designed to ever be machined. Car makers are trying to save weight everywhere they can, and rotors are one of those places. They're a lot thinner to start with, so machining them is pretty much out of the question, which is why most places refuse to turn them...they don't want the liability. If the rotors do not have a "Min Thickness" measurement cast into them, they are not meant to be machined. This is one reason new rotors are so much cheaper than they used to be....they are thinner to begin with.
There are some off-shore rotors that are junk, so you gotta be careful replacing them. Cheaper is not always better, especially when talking about brake components.
I bought new rotors for one of the Cadlacks last year....dam things were warped from the git go....   Just trying to save some dough, but it's bitin me on the bo-bo. I'll live with it, since my brake lathe is inaccessible at the time, but next time I put pads on, the rotors will get turned.