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 $180.82   12.698%   $203.7805   $203.78   $203.80   $203.80  
  Purchase #2 $98.85   4.138%   $102.9404   $102.94   $102.95   $102.90  
  Purchase #3 $60.69   11.437%   $67.6311   $67.63   $67.65   $67.60  
  Purchase #4 $111.33   13.629%   $126.5032   $126.50   $126.50   $126.50  
  Purchase #5 $221.71   10.229%   $244.3887   $244.39   $244.40   $244.40  
  Purchase #6 $19.79   11.756%   $22.1165   $22.12   $22.10   $22.10  
  Purchase #7 $6.85   10.114%   $7.5428   $7.54   $7.55   $7.50  
  7 purchase totals = $700.04       $774.9032   $774.90   $774.95   $774.80