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 $19.33   6.715%   $20.628   $20.63   $20.65   $20.60  
  Purchase #2 $5.62   5.198%   $5.9121   $5.91   $5.90   $5.90  
  Purchase #3 $245.92   5.594%   $259.6768   $259.68   $259.70   $259.70  
  Purchase #4 $8.45   11.355%   $9.4095   $9.41   $9.40   $9.40  
  Purchase #5 $73.61   11.845%   $82.3291   $82.33   $82.35   $82.30  
  Purchase #6 $110.52   3.644%   $114.5473   $114.55   $114.55   $114.50  
  Purchase #7 $144.98   5.433%   $152.8568   $152.86   $152.85   $152.90  
  7 purchase totals = $608.43       $645.3596   $645.37   $645.40   $645.30