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 $34.33   13.122%   $38.8348   $38.83   $38.85   $38.80  
  Purchase #2 $98.36   12.537%   $110.6914   $110.69   $110.70   $110.70  
  Purchase #3 $211.47   9.793%   $232.1793   $232.18   $232.20   $232.20  
  Purchase #4 $82.25   4.769%   $86.1725   $86.17   $86.15   $86.20  
  Purchase #5 $89.46   8.182%   $96.7796   $96.78   $96.80   $96.80  
  Purchase #6 $207.81   13.777%   $236.44   $236.44   $236.45   $236.40  
  Purchase #7 $199.88   10.333%   $220.5336   $220.53   $220.55   $220.50  
  7 purchase totals = $923.56       $1,021.6312   $1,021.62   $1,021.70   $1,021.60