Designing graphics for the lowest common denominator really means not making a site at all - only a small minority of the population have access to the internet so you 'd better cater for all those people that can't see it at all! My point here is regardless of where you draw your line you will be favouring some visitors, and not catering for others so don't get pushed into thinking that this line is set in cement - it's up to you to make the call.

Personally I don't stick to the web safe palette. Using it doesn't really gaurantee anything so why be so limited? You can't control all the variables so it seems pointless to get hung up on the web safe palette. The visitors with  old monitors, poorly configured, poor lighting.. well, not even the web safe palette will help them..

What about sticking to 256 colours? How many people are still surfing at 256? Surveys suggest there's still quite a few. But should you stick to 256 then? Given the abundance of hi-colour sites, people surfing at 256 colours must be used to a large part of the web looking pretty poor.

As we push the numbers of colours up chances are we'll be increasing the file size and it's this balancing act that seems to play the pivotal role in decision making, rather than the viewer and/or the web safe palette.

So rather than getting hung up on it, maybe a more realistic option is to design efficiently keeping to the smallest number of colours you really need. If there is a spec you wish to target e.g. windows running at 256 - then test it widely.

While it's important to have a site look ok across a range of machines and settings, the reality is that your site will always look best on your own machine, to your own eyes - It's just one of those things....

The real web safe palette to me is one that meets your requirements as the designer, is no larger than it needs to be and stands up to viewing exceptionally well on your target machines/settings and ok on others