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 $51.74   8.657%   $56.2191   $56.22   $56.20   $56.20  
  Purchase #2 $82.32   6.830%   $87.9425   $87.94   $87.95   $87.90  
  Purchase #3 $95.26   5.353%   $100.3593   $100.36   $100.35   $100.40  
  Purchase #4 $77.95   8.456%   $84.5415   $84.54   $84.55   $84.50  
  Purchase #5 $65.40   4.302%   $68.2135   $68.21   $68.20   $68.20  
  Purchase #6 $245.49   13.219%   $277.9413   $277.94   $277.95   $277.90  
  Purchase #7 $165.26   6.146%   $175.4169   $175.42   $175.40   $175.40  
  7 purchase totals = $783.42       $850.6341   $850.63   $850.60   $850.50