What is the ideal butter softness for baking cookies etc?

Ideal = 60F (16C) By the time butter reaches 70F (21C) it won't cream properly.

In the Kitchen Basics column on WFM Cooking, Susan shares some tips and a photo to demonstrate ideal softness. Basically, if you press your finger on the butter, it should leave a well, but the sides shouldn't cave in. http://nyc.wholefoodsmarketcooking.com/blog/3214

Soft enough that a butter knife easily slides through it (and spreads smoothly on a piece of soft bread), but still holds its stick shape...i.e., not so soft that it starts to look like a butter blob.

Generally speaking if you microwave a half pound of butter for 15-20 seconds you should have the right consistency

I'd like to hear what others say, but I'm pretty sure you don't want it too soft. If you are using recipes that "cream" the butter with sugar, I believe you want the butter to stay solid so that you incorporate air into the mix as you cream.
I usually don't let butter soften at all (impatient). I find that if I roughly chop the butter first, and maybe let it sit 5 minutes while I get other ingredients out, it creams really well. And I live in the Philadelphia area. Good question!

I do have a butter bell and it's fantastic but usually the quantity of butter I need for cookies, muffins and cakes is larger than what I keep in te bell.

Get a butter bell, keep the butter in it, with the water at about 17c or cool "room temp". The thermal efficiency of water will help buffet the temperature and protect your butter.