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 $27.39   9.431%   $29.9732   $29.97   $29.95   $30.00  
  Purchase #2 $21.54   3.403%   $22.273   $22.27   $22.25   $22.30  
  Purchase #3 $40.83   10.531%   $45.1298   $45.13   $45.15   $45.10  
  Purchase #4 $245.36   13.814%   $279.254   $279.25   $279.25   $279.30  
  Purchase #5 $125.95   10.150%   $138.7339   $138.73   $138.75   $138.70  
  Purchase #6 $117.97   13.469%   $133.8594   $133.86   $133.85   $133.90  
  Purchase #7 $82.49   4.125%   $85.8927   $85.89   $85.90   $85.90  
  7 purchase totals = $661.53       $735.1160   $735.10   $735.10   $735.20