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 $219.62   13.713%   $249.7365   $249.74   $249.75   $249.70  
  Purchase #2 $237.31   9.369%   $259.5436   $259.54   $259.55   $259.50  
  Purchase #3 $117.63   3.568%   $121.827   $121.83   $121.85   $121.80  
  Purchase #4 $36.52   11.163%   $40.5967   $40.60   $40.60   $40.60  
  Purchase #5 $83.73   5.786%   $88.5746   $88.57   $88.55   $88.60  
  Purchase #6 $127.47   10.808%   $141.247   $141.25   $141.25   $141.20  
  Purchase #7 $111.36   5.733%   $117.7443   $117.74   $117.75   $117.70  
  7 purchase totals = $933.64       $1,019.2697   $1,019.27   $1,019.30   $1,019.10