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 $27.70   3.641%   $28.7086   $28.71   $28.70   $28.70  
  Purchase #2 $12.48   10.869%   $13.8365   $13.84   $13.85   $13.80  
  Purchase #3 $125.55   3.379%   $129.7923   $129.79   $129.80   $129.80  
  Purchase #4 $169.59   8.744%   $184.4189   $184.42   $184.40   $184.40  
  Purchase #5 $123.80   12.129%   $138.8157   $138.82   $138.80   $138.80  
  Purchase #6 $9.32   3.683%   $9.6633   $9.66   $9.65   $9.70  
  Purchase #7 $140.89   11.793%   $157.5052   $157.51   $157.50   $157.50  
  7 purchase totals = $609.33       $662.7405   $662.75   $662.70   $662.70