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 $119.97   13.809%   $136.5367   $136.54   $136.55   $136.50  
  Purchase #2 $248.34   8.588%   $269.6674   $269.67   $269.65   $269.70  
  Purchase #3 $216.67   3.148%   $223.4908   $223.49   $223.50   $223.50  
  Purchase #4 $75.27   11.122%   $83.6415   $83.64   $83.65   $83.60  
  Purchase #5 $15.66   9.306%   $17.1173   $17.12   $17.10   $17.10  
  Purchase #6 $67.76   13.422%   $76.8547   $76.85   $76.85   $76.90  
  Purchase #7 $202.81   6.654%   $216.305   $216.31   $216.30   $216.30  
  7 purchase totals = $946.48       $1,023.6134   $1,023.62   $1,023.60   $1,023.60