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 $247.97   11.149%   $275.6162   $275.62   $275.60   $275.60  
  Purchase #2 $196.95   7.727%   $212.1683   $212.17   $212.15   $212.20  
  Purchase #3 $36.84   7.174%   $39.4829   $39.48   $39.50   $39.50  
  Purchase #4 $125.72   9.213%   $137.3026   $137.30   $137.30   $137.30  
  Purchase #5 $250.87   7.854%   $270.5733   $270.57   $270.55   $270.60  
  Purchase #6 $169.71   3.302%   $175.3138   $175.31   $175.30   $175.30  
  Purchase #7 $132.80   5.359%   $139.9168   $139.92   $139.90   $139.90  
  7 purchase totals = $1,160.86       $1,250.3739   $1,250.37   $1,250.30   $1,250.40