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 $18.77   8.498%   $20.3651   $20.37   $20.35   $20.40  
  Purchase #2 $28.89   6.621%   $30.8028   $30.80   $30.80   $30.80  
  Purchase #3 $158.41   12.147%   $177.6521   $177.65   $177.65   $177.70  
  Purchase #4 $143.46   9.867%   $157.6152   $157.62   $157.60   $157.60  
  Purchase #5 $66.38   9.771%   $72.866   $72.87   $72.85   $72.90  
  Purchase #6 $159.28   6.236%   $169.2127   $169.21   $169.20   $169.20  
  Purchase #7 $18.56   12.685%   $20.9143   $20.91   $20.90   $20.90  
  7 purchase totals = $593.75       $649.4282   $649.43   $649.35   $649.50