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 $244.77   12.420%   $275.1704   $275.17   $275.15   $275.20  
  Purchase #2 $103.93   9.455%   $113.7566   $113.76   $113.75   $113.80  
  Purchase #3 $191.76   4.231%   $199.8734   $199.87   $199.85   $199.90  
  Purchase #4 $127.28   11.792%   $142.2889   $142.29   $142.30   $142.30  
  Purchase #5 $84.68   8.106%   $91.5442   $91.54   $91.55   $91.50  
  Purchase #6 $73.91   7.176%   $79.2138   $79.21   $79.20   $79.20  
  Purchase #7 $165.62   12.535%   $186.3805   $186.38   $186.40   $186.40  
  7 purchase totals = $991.95       $1,088.2278   $1,088.22   $1,088.20   $1,088.30