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 $171.81   11.456%   $191.4926   $191.49   $191.50   $191.50  
  Purchase #2 $116.88   11.546%   $130.375   $130.38   $130.40   $130.40  
  Purchase #3 $9.96   10.744%   $11.0301   $11.03   $11.05   $11.00  
  Purchase #4 $35.69   12.462%   $40.1377   $40.14   $40.15   $40.10  
  Purchase #5 $108.36   11.716%   $121.0555   $121.06   $121.05   $121.10  
  Purchase #6 $123.65   13.526%   $140.3749   $140.37   $140.35   $140.40  
  Purchase #7 $74.52   12.313%   $83.6956   $83.70   $83.70   $83.70  
  7 purchase totals = $640.87       $718.1614   $718.17   $718.20   $718.20