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 $55.89   5.550%   $58.9919   $58.99   $59.00   $59.00  
  Purchase #2 $49.89   12.247%   $56   $56.00   $56.00   $56.00  
  Purchase #3 $199.88   13.552%   $226.9677   $226.97   $226.95   $227.00  
  Purchase #4 $133.51   9.798%   $146.5913   $146.59   $146.60   $146.60  
  Purchase #5 $14.61   11.271%   $16.2567   $16.26   $16.25   $16.30  
  Purchase #6 $243.30   13.651%   $276.5129   $276.51   $276.50   $276.50  
  Purchase #7 $141.81   3.201%   $146.3493   $146.35   $146.35   $146.30  
  7 purchase totals = $838.89       $927.6698   $927.67   $927.65   $927.70