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 $246.36   12.536%   $277.2437   $277.24   $277.25   $277.20  
  Purchase #2 $85.28   3.710%   $88.4439   $88.44   $88.45   $88.40  
  Purchase #3 $88.63   11.418%   $98.7498   $98.75   $98.75   $98.70  
  Purchase #4 $52.77   7.225%   $56.5826   $56.58   $56.60   $56.60  
  Purchase #5 $8.29   12.756%   $9.3475   $9.35   $9.35   $9.30  
  Purchase #6 $128.92   7.871%   $139.0673   $139.07   $139.05   $139.10  
  Purchase #7 $167.72   6.480%   $178.5883   $178.59   $178.60   $178.60  
  7 purchase totals = $777.97       $848.0231   $848.02   $848.05   $847.90