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 $18.76   7.694%   $20.2034   $20.20   $20.20   $20.20  
  Purchase #2 $131.99   7.639%   $142.0727   $142.07   $142.05   $142.10  
  Purchase #3 $67.68   12.221%   $75.9512   $75.95   $75.95   $76.00  
  Purchase #4 $33.63   12.110%   $37.7026   $37.70   $37.70   $37.70  
  Purchase #5 $39.40   10.124%   $43.3889   $43.39   $43.40   $43.40  
  Purchase #6 $82.94   5.363%   $87.3881   $87.39   $87.40   $87.40  
  Purchase #7 $99.35   3.592%   $102.9187   $102.92   $102.90   $102.90  
  7 purchase totals = $473.75       $509.6256   $509.62   $509.60   $509.70