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 $19.77   10.582%   $21.8621   $21.86   $21.85   $21.90  
  Purchase #2 $83.66   3.500%   $86.5881   $86.59   $86.60   $86.60  
  Purchase #3 $25.63   6.470%   $27.2883   $27.29   $27.30   $27.30  
  Purchase #4 $124.33   10.739%   $137.6818   $137.68   $137.70   $137.70  
  Purchase #5 $220.68   8.428%   $239.2789   $239.28   $239.30   $239.30  
  Purchase #6 $210.72   11.613%   $235.1909   $235.19   $235.20   $235.20  
  Purchase #7 $81.90   9.856%   $89.9721   $89.97   $89.95   $90.00  
  7 purchase totals = $766.69       $837.8622   $837.86   $837.90   $838.00