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 $242.48   12.793%   $273.5005   $273.50   $273.50   $273.50  
  Purchase #2 $51.53   12.362%   $57.9001   $57.90   $57.90   $57.90  
  Purchase #3 $22.78   12.369%   $25.5977   $25.60   $25.60   $25.60  
  Purchase #4 $219.32   8.218%   $237.3437   $237.34   $237.35   $237.30  
  Purchase #5 $6.33   8.648%   $6.8774   $6.88   $6.90   $6.90  
  Purchase #6 $32.82   8.794%   $35.7062   $35.71   $35.70   $35.70  
  Purchase #7 $158.34   5.823%   $167.5601   $167.56   $167.55   $167.60  
  7 purchase totals = $733.60       $804.4857   $804.49   $804.50   $804.50