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 $28.48   11.758%   $31.8287   $31.83   $31.85   $31.80  
  Purchase #2 $31.32   9.102%   $34.1707   $34.17   $34.15   $34.20  
  Purchase #3 $76.85   5.282%   $80.9092   $80.91   $80.90   $80.90  
  Purchase #4 $109.95   7.854%   $118.5855   $118.59   $118.60   $118.60  
  Purchase #5 $43.75   10.120%   $48.1775   $48.18   $48.20   $48.20  
  Purchase #6 $18.71   7.403%   $20.0951   $20.10   $20.10   $20.10  
  Purchase #7 $191.44   13.286%   $216.8747   $216.87   $216.85   $216.90  
  7 purchase totals = $500.50       $550.6414   $550.65   $550.65   $550.70