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 $242.77   6.111%   $257.6057   $257.61   $257.60   $257.60  
  Purchase #2 $52.52   5.624%   $55.4737   $55.47   $55.45   $55.50  
  Purchase #3 $154.55   6.197%   $164.1275   $164.13   $164.15   $164.10  
  Purchase #4 $192.34   6.504%   $204.8498   $204.85   $204.85   $204.80  
  Purchase #5 $238.33   11.540%   $265.8333   $265.83   $265.85   $265.80  
  Purchase #6 $105.33   9.308%   $115.1341   $115.13   $115.15   $115.10  
  Purchase #7 $248.68   4.730%   $260.4426   $260.44   $260.45   $260.40  
  7 purchase totals = $1,234.52       $1,323.4667   $1,323.46   $1,323.50   $1,323.30