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 $151.25   10.195%   $166.6699   $166.67   $166.65   $166.70  
  Purchase #2 $76.70   6.354%   $81.5735   $81.57   $81.55   $81.60  
  Purchase #3 $246.77   3.200%   $254.6666   $254.67   $254.65   $254.70  
  Purchase #4 $186.73   10.249%   $205.868   $205.87   $205.85   $205.90  
  Purchase #5 $51.79   10.116%   $57.0291   $57.03   $57.05   $57.00  
  Purchase #6 $157.41   3.227%   $162.4896   $162.49   $162.50   $162.50  
  Purchase #7 $73.38   10.746%   $81.2654   $81.27   $81.25   $81.30  
  7 purchase totals = $944.03       $1,009.5621   $1,009.57   $1,009.50   $1,009.70