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 $123.94   10.161%   $136.5335   $136.53   $136.55   $136.50  
  Purchase #2 $114.86   8.539%   $124.6679   $124.67   $124.65   $124.70  
  Purchase #3 $132.97   11.421%   $148.1565   $148.16   $148.15   $148.20  
  Purchase #4 $84.48   5.147%   $88.8282   $88.83   $88.85   $88.80  
  Purchase #5 $153.48   13.315%   $173.9159   $173.92   $173.90   $173.90  
  Purchase #6 $131.81   5.404%   $138.933   $138.93   $138.95   $138.90  
  Purchase #7 $217.76   7.159%   $233.3494   $233.35   $233.35   $233.30  
  7 purchase totals = $959.30       $1,044.3844   $1,044.39   $1,044.40   $1,044.30