Same issue here.  Annoying I cant' view subtotal (before tax) when invoicing. 

(MY TEMP SOLUTION)

- Click the "Tax" drop down and choose no tax, to see the amount without tax.. 
- Then change it back to tax.  

- Another alternative, is to just use a calculator to manually subtract the tax from the total..but that takes longer to key all in.. So I just do the sales tax thing.