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 $242.79   6.389%   $258.3019   $258.30   $258.30   $258.30  
  Purchase #2 $90.87   11.308%   $101.1456   $101.15   $101.15   $101.10  
  Purchase #3 $61.75   5.819%   $65.3432   $65.34   $65.35   $65.30  
  Purchase #4 $234.89   9.771%   $257.8411   $257.84   $257.85   $257.80  
  Purchase #5 $108.89   12.255%   $122.2345   $122.23   $122.25   $122.20  
  Purchase #6 $233.52   5.624%   $246.6532   $246.65   $246.65   $246.70  
  Purchase #7 $34.30   3.226%   $35.4065   $35.41   $35.40   $35.40  
  7 purchase totals = $1,007.01       $1,086.9260   $1,086.92   $1,086.95   $1,086.80