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 $18.36   12.778%   $20.706   $20.71   $20.70   $20.70  
  Purchase #2 $128.99   12.502%   $145.1163   $145.12   $145.10   $145.10  
  Purchase #3 $127.32   6.206%   $135.2215   $135.22   $135.20   $135.20  
  Purchase #4 $82.31   3.716%   $85.3686   $85.37   $85.35   $85.40  
  Purchase #5 $188.62   13.626%   $214.3214   $214.32   $214.30   $214.30  
  Purchase #6 $138.84   4.562%   $145.1739   $145.17   $145.15   $145.20  
  Purchase #7 $27.28   12.254%   $30.6229   $30.62   $30.60   $30.60  
  7 purchase totals = $711.72       $776.5306   $776.53   $776.40   $776.50