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 $246.52   11.763%   $275.5181   $275.52   $275.50   $275.50  
  Purchase #2 $176.52   12.341%   $198.3043   $198.30   $198.30   $198.30  
  Purchase #3 $24.74   11.149%   $27.4983   $27.50   $27.50   $27.50  
  Purchase #4 $119.42   12.285%   $134.0907   $134.09   $134.10   $134.10  
  Purchase #5 $156.54   3.444%   $161.9312   $161.93   $161.95   $161.90  
  Purchase #6 $179.66   6.438%   $191.2265   $191.23   $191.25   $191.20  
  Purchase #7 $10.41   9.432%   $11.3919   $11.39   $11.40   $11.40  
  7 purchase totals = $913.81       $999.9610   $999.96   $1,000.00   $999.90