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 $22.40   7.676%   $24.1194   $24.12   $24.10   $24.10  
  Purchase #2 $71.42   10.415%   $78.8584   $78.86   $78.85   $78.90  
  Purchase #3 $137.90   6.572%   $146.9628   $146.96   $146.95   $147.00  
  Purchase #4 $145.63   8.337%   $157.7712   $157.77   $157.75   $157.80  
  Purchase #5 $102.95   4.692%   $107.7804   $107.78   $107.80   $107.80  
  Purchase #6 $76.91   10.849%   $85.254   $85.25   $85.25   $85.30  
  Purchase #7 $35.93   8.732%   $39.0674   $39.07   $39.05   $39.10  
  7 purchase totals = $593.14       $639.8136   $639.81   $639.75   $640.00