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 $238.78   8.472%   $259.0094   $259.01   $259.00   $259.00  
  Purchase #2 $179.53   9.576%   $196.7218   $196.72   $196.70   $196.70  
  Purchase #3 $205.55   13.330%   $232.9498   $232.95   $232.95   $232.90  
  Purchase #4 $142.35   11.616%   $158.8854   $158.89   $158.90   $158.90  
  Purchase #5 $193.98   10.665%   $214.668   $214.67   $214.65   $214.70  
  Purchase #6 $218.86   10.818%   $242.5363   $242.54   $242.55   $242.50  
  Purchase #7 $62.92   7.493%   $67.6346   $67.63   $67.65   $67.60  
  7 purchase totals = $1,241.97       $1,372.4053   $1,372.41   $1,372.40   $1,372.30