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 $16.70   6.736%   $17.8249   $17.82   $17.80   $17.80  
  Purchase #2 $74.54   9.352%   $81.511   $81.51   $81.50   $81.50  
  Purchase #3 $209.61   3.604%   $217.1643   $217.16   $217.15   $217.20  
  Purchase #4 $196.34   13.191%   $222.2392   $222.24   $222.25   $222.20  
  Purchase #5 $65.39   13.578%   $74.2687   $74.27   $74.25   $74.30  
  Purchase #6 $126.79   13.388%   $143.7646   $143.76   $143.75   $143.80  
  Purchase #7 $127.59   4.126%   $132.8544   $132.85   $132.85   $132.90  
  7 purchase totals = $816.96       $889.6271   $889.61   $889.55   $889.70