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 $16.84   8.373%   $18.25   $18.25   $18.25   $18.30  
  Purchase #2 $16.65   5.216%   $17.5185   $17.52   $17.50   $17.50  
  Purchase #3 $132.60   11.214%   $147.4698   $147.47   $147.45   $147.50  
  Purchase #4 $93.38   4.255%   $97.3533   $97.35   $97.35   $97.40  
  Purchase #5 $76.84   6.370%   $81.7347   $81.73   $81.75   $81.70  
  Purchase #6 $80.52   13.586%   $91.4594   $91.46   $91.45   $91.50  
  Purchase #7 $222.32   4.376%   $232.0487   $232.05   $232.05   $232.00  
  7 purchase totals = $639.15       $685.8344   $685.83   $685.80   $685.90