The consideration isn't just one of smoke. There was likely some damage due to heat. Hard drives would be particularly sensitive to this. If just one soldered contact on the motherboard got hot enough to bridge another contact, you could get all kinds of bizarre issues (including damaging data when the computer is started up). You REALLY need a known good system to pull the data off the hard drives. You also need to consider that the power supplies may have been damaged even if the logic boards weren't. If the transformer melted you could send AC through your system and cause another fire.
Also, denatured alcohol still has a huge amount of water in it. You may want to use wood grain alcohol. I believe it is 100% alcohol (not 20 to 30% water like denatured).