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 $124.54   8.206%   $134.7598   $134.76   $134.75   $134.80  
  Purchase #2 $122.53   13.190%   $138.6917   $138.69   $138.70   $138.70  
  Purchase #3 $249.66   3.858%   $259.2919   $259.29   $259.30   $259.30  
  Purchase #4 $186.56   9.496%   $204.2757   $204.28   $204.30   $204.30  
  Purchase #5 $159.26   3.614%   $165.0157   $165.02   $165.00   $165.00  
  Purchase #6 $184.33   11.389%   $205.3233   $205.32   $205.30   $205.30  
  Purchase #7 $133.57   6.764%   $142.6047   $142.60   $142.60   $142.60  
  7 purchase totals = $1,160.45       $1,249.9628   $1,249.96   $1,249.95   $1,250.00