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 $227.55   11.522%   $253.7683   $253.77   $253.75   $253.80  
  Purchase #2 $165.47   4.673%   $173.2024   $173.20   $173.20   $173.20  
  Purchase #3 $70.77   6.703%   $75.5137   $75.51   $75.50   $75.50  
  Purchase #4 $101.43   9.853%   $111.4239   $111.42   $111.40   $111.40  
  Purchase #5 $131.81   6.789%   $140.7586   $140.76   $140.75   $140.80  
  Purchase #6 $231.30   4.685%   $242.1364   $242.14   $242.15   $242.10  
  Purchase #7 $23.25   5.770%   $24.5915   $24.59   $24.60   $24.60  
  7 purchase totals = $951.58       $1,021.3948   $1,021.39   $1,021.35   $1,021.40