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 $76.93   10.261%   $84.8238   $84.82   $84.80   $84.80  
  Purchase #2 $25.41   8.707%   $27.6224   $27.62   $27.60   $27.60  
  Purchase #3 $206.66   8.496%   $224.2178   $224.22   $224.20   $224.20  
  Purchase #4 $237.31   12.686%   $267.4151   $267.42   $267.40   $267.40  
  Purchase #5 $121.41   13.606%   $137.929   $137.93   $137.95   $137.90  
  Purchase #6 $91.63   7.826%   $98.801   $98.80   $98.80   $98.80  
  Purchase #7 $124.25   11.461%   $138.4903   $138.49   $138.50   $138.50  
  7 purchase totals = $883.60       $979.2994   $979.30   $979.25   $979.20