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 $71.58   11.457%   $79.7809   $79.78   $79.80   $79.80  
  Purchase #2 $84.95   8.196%   $91.9125   $91.91   $91.90   $91.90  
  Purchase #3 $229.76   10.103%   $252.9727   $252.97   $252.95   $253.00  
  Purchase #4 $63.64   4.373%   $66.423   $66.42   $66.40   $66.40  
  Purchase #5 $231.71   6.618%   $247.0446   $247.04   $247.05   $247.00  
  Purchase #6 $83.32   6.382%   $88.6375   $88.64   $88.65   $88.60  
  Purchase #7 $180.75   8.655%   $196.3939   $196.39   $196.40   $196.40  
  7 purchase totals = $945.71       $1,023.1651   $1,023.15   $1,023.15   $1,023.10