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 $226.73   12.273%   $254.5566   $254.56   $254.55   $254.60  
  Purchase #2 $226.49   13.144%   $256.2598   $256.26   $256.25   $256.30  
  Purchase #3 $112.29   12.547%   $126.379   $126.38   $126.40   $126.40  
  Purchase #4 $60.71   3.556%   $62.8688   $62.87   $62.85   $62.90  
  Purchase #5 $134.81   9.805%   $148.0281   $148.03   $148.05   $148.00  
  Purchase #6 $61.79   10.793%   $68.459   $68.46   $68.45   $68.50  
  Purchase #7 $175.66   12.644%   $197.8705   $197.87   $197.85   $197.90  
  7 purchase totals = $998.48       $1,114.4218   $1,114.43   $1,114.40   $1,114.60