Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $58.80   13.304%   $66.6228   $66.62   $66.60   $66.60  
  Purchase #2 $165.37   5.686%   $174.7729   $174.77   $174.75   $174.80  
  Purchase #3 $107.81   3.152%   $111.2082   $111.21   $111.20   $111.20  
  Purchase #4 $108.49   4.492%   $113.3634   $113.36   $113.35   $113.40  
  Purchase #5 $97.84   10.216%   $107.8353   $107.84   $107.85   $107.80  
  Purchase #6 $25.26   12.707%   $28.4698   $28.47   $28.45   $28.50  
  Purchase #7 $53.79   13.292%   $60.9398   $60.94   $60.95   $60.90  
  7 purchase totals = $617.36       $663.2122   $663.21   $663.15   $663.20