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 $241.67   12.454%   $271.7676   $271.77   $271.75   $271.80  
  Purchase #2 $54.95   11.602%   $61.3253   $61.33   $61.35   $61.30  
  Purchase #3 $71.34   3.639%   $73.9361   $73.94   $73.95   $73.90  
  Purchase #4 $151.97   11.325%   $169.1806   $169.18   $169.20   $169.20  
  Purchase #5 $55.92   7.612%   $60.1766   $60.18   $60.20   $60.20  
  Purchase #6 $88.65   10.798%   $98.2224   $98.22   $98.20   $98.20  
  Purchase #7 $103.57   12.188%   $116.1931   $116.19   $116.20   $116.20  
  7 purchase totals = $768.07       $850.8017   $850.81   $850.85   $850.80