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 $20.79   3.287%   $21.4734   $21.47   $21.45   $21.50  
  Purchase #2 $95.31   9.600%   $104.4598   $104.46   $104.45   $104.50  
  Purchase #3 $219.64   10.119%   $241.8654   $241.87   $241.85   $241.90  
  Purchase #4 $104.63   8.628%   $113.6575   $113.66   $113.65   $113.70  
  Purchase #5 $81.83   6.696%   $87.3093   $87.31   $87.30   $87.30  
  Purchase #6 $167.69   6.720%   $178.9588   $178.96   $178.95   $179.00  
  Purchase #7 $132.47   4.742%   $138.7517   $138.75   $138.75   $138.80  
  7 purchase totals = $822.36       $886.4759   $886.48   $886.40   $886.70