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 $19.80   6.790%   $21.1444   $21.14   $21.15   $21.10  
  Purchase #2 $10.98   12.705%   $12.375   $12.38   $12.40   $12.40  
  Purchase #3 $65.44   4.158%   $68.161   $68.16   $68.15   $68.20  
  Purchase #4 $64.26   13.531%   $72.955   $72.96   $72.95   $73.00  
  Purchase #5 $84.30   12.530%   $94.8628   $94.86   $94.85   $94.90  
  Purchase #6 $221.77   12.805%   $250.1676   $250.17   $250.15   $250.20  
  Purchase #7 $9.92   3.249%   $10.2423   $10.24   $10.25   $10.20  
  7 purchase totals = $476.47       $529.9081   $529.91   $529.90   $530.00