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 $136.51   12.117%   $153.0509   $153.05   $153.05   $153.10  
  Purchase #2 $249.46   3.314%   $257.7271   $257.73   $257.75   $257.70  
  Purchase #3 $10.44   11.290%   $11.6187   $11.62   $11.60   $11.60  
  Purchase #4 $129.61   13.418%   $147.0011   $147.00   $147.00   $147.00  
  Purchase #5 $35.94   10.523%   $39.722   $39.72   $39.70   $39.70  
  Purchase #6 $129.53   12.629%   $145.8883   $145.89   $145.90   $145.90  
  Purchase #7 $92.48   3.257%   $95.4921   $95.49   $95.50   $95.50  
  7 purchase totals = $783.97       $850.5002   $850.50   $850.50   $850.50