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 $108.28   10.346%   $119.4826   $119.48   $119.50   $119.50  
  Purchase #2 $66.88   3.793%   $69.4168   $69.42   $69.40   $69.40  
  Purchase #3 $114.59   11.378%   $127.6281   $127.63   $127.65   $127.60  
  Purchase #4 $229.79   5.856%   $243.2465   $243.25   $243.25   $243.20  
  Purchase #5 $223.68   11.739%   $249.9378   $249.94   $249.95   $249.90  
  Purchase #6 $98.48   12.173%   $110.468   $110.47   $110.45   $110.50  
  Purchase #7 $47.77   5.282%   $50.2932   $50.29   $50.30   $50.30  
  7 purchase totals = $889.47       $970.4730   $970.48   $970.50   $970.40