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 $27.97   12.628%   $31.5021   $31.50   $31.50   $31.50  
  Purchase #2 $11.73   5.553%   $12.3814   $12.38   $12.40   $12.40  
  Purchase #3 $172.79   6.574%   $184.1492   $184.15   $184.15   $184.10  
  Purchase #4 $134.57   4.837%   $141.0792   $141.08   $141.10   $141.10  
  Purchase #5 $247.91   9.301%   $270.9681   $270.97   $270.95   $271.00  
  Purchase #6 $249.27   9.330%   $272.5269   $272.53   $272.55   $272.50  
  Purchase #7 $209.83   8.232%   $227.1032   $227.10   $227.10   $227.10  
  7 purchase totals = $1,054.07       $1,139.7101   $1,139.71   $1,139.75   $1,139.70