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 $250.62   13.636%   $284.7945   $284.79   $284.80   $284.80  
  Purchase #2 $117.30   7.751%   $126.3919   $126.39   $126.40   $126.40  
  Purchase #3 $65.93   12.872%   $74.4165   $74.42   $74.40   $74.40  
  Purchase #4 $40.72   13.180%   $46.0869   $46.09   $46.10   $46.10  
  Purchase #5 $186.65   13.488%   $211.8254   $211.83   $211.85   $211.80  
  Purchase #6 $66.87   5.511%   $70.5552   $70.56   $70.55   $70.60  
  Purchase #7 $199.44   8.616%   $216.6238   $216.62   $216.60   $216.60  
  7 purchase totals = $927.53       $1,030.6942   $1,030.70   $1,030.70   $1,030.70