Wow so much chatter about default skills and their values - most excitement that we have ad in this thread for some time.
OK on to business:
As far as having skills with automatic Default values when set to zero points, it just cannot be done.
There are some 472 skills based on 19 or so different Attributes and modifiers, and 47 which do have a default at all, and a good number that only default to other skills (and multiple ones at that). Some of attribute ones default to a choice of attributes, in fact look at Stealth as it defaults to IQ-5 or DX-5 and is a DX based skill. If your IQ happens to be higher than your DX, then defaulting to IQ is better - until such time as you spend a point on Stealth.
Without a compendium, there is now way the sheet can determine the proper Default value. The best way, as mentioned before. is to enter the skill in and use the MOD to adjust them with negative modifier. Most people are using either GCA or GCS to create their PCs and entering the data in manually. so that is an easy calculation to make.
In the following example I have a PC with DX 13 and IQ14.. I am using the Default of Stealth which is either defaults to IQ-5 or DX-5. Well IQ (14)-5=9 and DX (13) - 5 = 8. Therefore I choose to default from IQ. I enter this as shown below.

I set everything normally, except I put in a -2 modifier to make thee skill level 9.(whch is my IQ (14) - 5,
The main issue is if you change your IQ or DX. If you change the DX then the attribute value will change thus you will need to adjust the MOD to get it back to the default value.
If your IQ changes, then you need to again adjust the MOD to set the new value.
A bit wonky I admit but for no =w that is the best we can do.
A possible solution it is to have a Default Attribute setting and 'Linking' it to either an attribute or another skill in the skill list, then a Modifier. We would have to put is some logic to determine if a point is spent in the skill, then delete the value and linking that were set for the default and hide the Default attribute from being used. So much to do, so little time.
So we have added this to our research list.