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 $244.31   6.847%   $261.0379   $261.04   $261.05   $261.00  
  Purchase #2 $88.31   10.597%   $97.6682   $97.67   $97.65   $97.70  
  Purchase #3 $48.48   8.574%   $52.6367   $52.64   $52.65   $52.60  
  Purchase #4 $103.90   6.140%   $110.2795   $110.28   $110.30   $110.30  
  Purchase #5 $163.82   6.527%   $174.5125   $174.51   $174.50   $174.50  
  Purchase #6 $241.68   4.583%   $252.7562   $252.76   $252.75   $252.80  
  Purchase #7 $28.84   3.290%   $29.7888   $29.79   $29.80   $29.80  
  7 purchase totals = $919.34       $978.6798   $978.69   $978.70   $978.70