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 $230.61   13.459%   $261.6478   $261.65   $261.65   $261.60  
  Purchase #2 $111.35   10.860%   $123.4426   $123.44   $123.45   $123.40  
  Purchase #3 $25.32   13.573%   $28.7567   $28.76   $28.75   $28.80  
  Purchase #4 $125.74   10.261%   $138.6422   $138.64   $138.65   $138.60  
  Purchase #5 $196.79   5.519%   $207.6508   $207.65   $207.65   $207.70  
  Purchase #6 $165.90   8.214%   $179.527   $179.53   $179.55   $179.50  
  Purchase #7 $43.59   3.719%   $45.2111   $45.21   $45.20   $45.20  
  7 purchase totals = $899.30       $984.8782   $984.88   $984.90   $984.80