Ok, so we are NOT talking about the nylock nut which you can see if you open the bonnet and look at the top of the damper rod? This is the nut which you remove to allow the strut to be removed from the vehicle and does not retain the spring itself. If you've removed the strut from the vehicle the nut you can now see on the damper rod is a rather larger nut and is not a nylock type. It also has a small lip on it's underside which helps to centre the bearing in the rubber top mount with the damper rod. I guess it's this nut you are concerned about?
There was no washer involved with this nut on either my boy's Punto or my Panda, indeed I would think a washer would interfere with the little lip of the nut fitting the top mount bearing?
As regards slackening this second, larger, nut without compressing the spring? Hmm? Well, thinking about it, the weight of the vehicle is being transferred from the vehicle body to the top mount and through it's bearing to the top spring retaining plate. This is tending to compress the spring as it bears against the bottom spring pan (part of the strut). Soo, as long as the vehicle is on level ground and you're not jacking it up at all, you probably could remove both nuts and "get away with it" However I wouldn't! A lifetime of working with vehicle springs has instilled a very, very healthy respect for them in me. Of course if the now unrestrained damper rod disappears down through the top mount while you've got the nuts off that's going to be real "fun" trying to get it back through the hole with an uncontrolled spring sitting there just waiting to "bite" you!
So, although you probably could do it, I wouldn't. And anyway, I don't think there's a washer under that big nut anyway? This "How to" video doesn't seem to show one: - I know it's a Panda but the 500 is the same isn't it?
