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 $231.71   10.811%   $256.7602   $256.76   $256.75   $256.80  
  Purchase #2 $127.25   13.837%   $144.8576   $144.86   $144.85   $144.90  
  Purchase #3 $147.82   4.655%   $154.701   $154.70   $154.70   $154.70  
  Purchase #4 $124.74   11.265%   $138.792   $138.79   $138.80   $138.80  
  Purchase #5 $158.82   4.612%   $166.1448   $166.14   $166.15   $166.10  
  Purchase #6 $249.72   12.865%   $281.8465   $281.85   $281.85   $281.80  
  Purchase #7 $19.61   4.313%   $20.4558   $20.46   $20.45   $20.50  
  7 purchase totals = $1,059.67       $1,163.5579   $1,163.56   $1,163.55   $1,163.60