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 $13.35   12.538%   $15.0238   $15.02   $15.00   $15.00  
  Purchase #2 $21.29   5.152%   $22.3869   $22.39   $22.40   $22.40  
  Purchase #3 $16.82   11.598%   $18.7708   $18.77   $18.75   $18.80  
  Purchase #4 $250.31   13.169%   $283.2733   $283.27   $283.25   $283.30  
  Purchase #5 $133.25   4.516%   $139.2676   $139.27   $139.25   $139.30  
  Purchase #6 $75.41   9.536%   $82.6011   $82.60   $82.60   $82.60  
  Purchase #7 $157.81   13.308%   $178.8114   $178.81   $178.80   $178.80  
  7 purchase totals = $668.24       $740.1349   $740.13   $740.05   $740.20