...if you've got rapid boiling, you're gonna have damage. This is controled by the amount of salt in the water. Also do it in stages, a couple minutes, take it out and brush off the black residue that forms with a toothbrush. Repeat until you've got it close to where you want it. I'll also rotate the position of the coin, front and back, top and bottom in relation to the stainless steel spoon (anode) to get an even process. I watch the shine on the highlights develope and try to quit one step too little instead one step too much -- a judgement call.
Done like this, the electrolysis will eat the corrosion before it eats the copper and fresh etching of the surface will be minimal. Of course if there is deep pitting, it is going to leave holes, and if the detail has been corroded away, then there will not be much left to look at...
When I am finished, I'll take back any shine by rubbing it with a little bit of coin darkener, which is a sulfur based oil and will turn the coin a chocolately brown and give it an aged look again.
I've heard over and over again NOT to clean coins, and if I find something I think is special, I'll drop down to the local coin shop before doing anything. As well, electrolysis will completely destroy a nice green patina, and I would rather have a little corrosion than disturb this natural finish.
But for the caked and crusty ones... the coins I have are for my own collection and they show a lot better if you can see what they are instead of being just crusty unidentified green disks. <img src="/metal/html/shrug.gif" border=0 width=37 height=15 alt=":shrug">