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 $39.81   6.421%   $42.3662   $42.37   $42.35   $42.40  
  Purchase #2 $150.68   7.577%   $162.097   $162.10   $162.10   $162.10  
  Purchase #3 $133.48   8.652%   $145.0287   $145.03   $145.05   $145.00  
  Purchase #4 $19.98   3.562%   $20.6917   $20.69   $20.70   $20.70  
  Purchase #5 $5.86   9.207%   $6.3995   $6.40   $6.40   $6.40  
  Purchase #6 $74.58   6.157%   $79.1719   $79.17   $79.15   $79.20  
  Purchase #7 $52.68   9.429%   $57.6472   $57.65   $57.65   $57.60  
  7 purchase totals = $477.07       $513.4022   $513.41   $513.40   $513.40