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 $21.28   3.173%   $21.9552   $21.96   $21.95   $22.00  
  Purchase #2 $54.33   12.309%   $61.0175   $61.02   $61.00   $61.00  
  Purchase #3 $107.94   6.233%   $114.6679   $114.67   $114.65   $114.70  
  Purchase #4 $244.43   11.404%   $272.3048   $272.30   $272.30   $272.30  
  Purchase #5 $58.70   13.551%   $66.6544   $66.65   $66.65   $66.70  
  Purchase #6 $7.59   13.490%   $8.6139   $8.61   $8.60   $8.60  
  Purchase #7 $86.51   8.437%   $93.8088   $93.81   $93.80   $93.80  
  7 purchase totals = $580.78       $639.0225   $639.02   $638.95   $639.10