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 $238.57   4.787%   $249.9903   $249.99   $250.00   $250.00  
  Purchase #2 $223.43   10.104%   $246.0054   $246.01   $246.00   $246.00  
  Purchase #3 $243.63   9.702%   $267.267   $267.27   $267.25   $267.30  
  Purchase #4 $37.46   5.444%   $39.4993   $39.50   $39.50   $39.50  
  Purchase #5 $119.54   12.692%   $134.712   $134.71   $134.70   $134.70  
  Purchase #6 $249.45   9.600%   $273.3972   $273.40   $273.40   $273.40  
  Purchase #7 $147.36   4.664%   $154.2329   $154.23   $154.25   $154.20  
  7 purchase totals = $1,259.44       $1,365.1041   $1,365.11   $1,365.10   $1,365.10