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 $246.55   10.534%   $272.5216   $272.52   $272.50   $272.50  
  Purchase #2 $182.74   10.175%   $201.3338   $201.33   $201.35   $201.30  
  Purchase #3 $181.28   10.190%   $199.7524   $199.75   $199.75   $199.80  
  Purchase #4 $77.96   8.743%   $84.776   $84.78   $84.80   $84.80  
  Purchase #5 $22.67   4.667%   $23.728   $23.73   $23.75   $23.70  
  Purchase #6 $27.71   5.225%   $29.1578   $29.16   $29.15   $29.20  
  Purchase #7 $65.42   4.529%   $68.3829   $68.38   $68.40   $68.40  
  7 purchase totals = $804.33       $879.6525   $879.65   $879.70   $879.70