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 $8.64   3.838%   $8.9716   $8.97   $8.95   $9.00  
  Purchase #2 $18.51   11.190%   $20.5813   $20.58   $20.60   $20.60  
  Purchase #3 $85.72   5.282%   $90.2477   $90.25   $90.25   $90.20  
  Purchase #4 $18.98   8.828%   $20.6556   $20.66   $20.65   $20.70  
  Purchase #5 $148.72   7.841%   $160.3811   $160.38   $160.40   $160.40  
  Purchase #6 $67.65   5.657%   $71.477   $71.48   $71.50   $71.50  
  Purchase #7 $188.89   9.414%   $206.6721   $206.67   $206.65   $206.70  
  7 purchase totals = $537.11       $578.9864   $578.99   $579.00   $579.10