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 $216.59   10.117%   $238.5024   $238.50   $238.50   $238.50  
  Purchase #2 $165.88   10.304%   $182.9723   $182.97   $182.95   $183.00  
  Purchase #3 $65.50   4.832%   $68.665   $68.67   $68.65   $68.70  
  Purchase #4 $133.37   4.528%   $139.409   $139.41   $139.40   $139.40  
  Purchase #5 $177.85   7.672%   $191.4947   $191.49   $191.50   $191.50  
  Purchase #6 $124.49   4.132%   $129.6339   $129.63   $129.65   $129.60  
  Purchase #7 $85.81   3.536%   $88.8442   $88.84   $88.85   $88.80  
  7 purchase totals = $969.49       $1,039.5215   $1,039.51   $1,039.50   $1,039.50