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 $18.89   6.230%   $20.0668   $20.07   $20.05   $20.10  
  Purchase #2 $22.62   3.237%   $23.3522   $23.35   $23.35   $23.40  
  Purchase #3 $5.55   12.694%   $6.2545   $6.25   $6.25   $6.30  
  Purchase #4 $129.89   11.429%   $144.7351   $144.74   $144.75   $144.70  
  Purchase #5 $139.64   9.662%   $153.132   $153.13   $153.15   $153.10  
  Purchase #6 $107.31   10.551%   $118.6323   $118.63   $118.65   $118.60  
  Purchase #7 $115.80   12.315%   $130.0608   $130.06   $130.05   $130.10  
  7 purchase totals = $539.70       $596.2337   $596.23   $596.25   $596.30