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 $124.52   11.647%   $139.0228   $139.02   $139.00   $139.00  
  Purchase #2 $35.83   5.207%   $37.6957   $37.70   $37.70   $37.70  
  Purchase #3 $156.59   13.585%   $177.8628   $177.86   $177.85   $177.90  
  Purchase #4 $159.31   7.798%   $171.733   $171.73   $171.75   $171.70  
  Purchase #5 $164.34   13.254%   $186.1216   $186.12   $186.10   $186.10  
  Purchase #6 $61.87   10.356%   $68.2773   $68.28   $68.30   $68.30  
  Purchase #7 $11.45   12.674%   $12.9012   $12.90   $12.90   $12.90  
  7 purchase totals = $713.91       $793.6144   $793.61   $793.60   $793.60