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 $60.34   13.325%   $68.3803   $68.38   $68.40   $68.40  
  Purchase #2 $14.73   7.732%   $15.8689   $15.87   $15.85   $15.90  
  Purchase #3 $132.73   4.549%   $138.7679   $138.77   $138.75   $138.80  
  Purchase #4 $93.42   6.455%   $99.4503   $99.45   $99.45   $99.50  
  Purchase #5 $153.68   7.773%   $165.6255   $165.63   $165.65   $165.60  
  Purchase #6 $173.30   3.702%   $179.7156   $179.72   $179.70   $179.70  
  Purchase #7 $47.35   7.683%   $50.9879   $50.99   $51.00   $51.00  
  7 purchase totals = $675.55       $718.7964   $718.81   $718.80   $718.90