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 $93.79   6.867%   $100.2306   $100.23   $100.25   $100.20  
  Purchase #2 $16.48   10.188%   $18.159   $18.16   $18.15   $18.20  
  Purchase #3 $139.55   13.493%   $158.3795   $158.38   $158.40   $158.40  
  Purchase #4 $55.40   11.460%   $61.7488   $61.75   $61.75   $61.70  
  Purchase #5 $111.49   3.840%   $115.7712   $115.77   $115.75   $115.80  
  Purchase #6 $25.27   5.331%   $26.6171   $26.62   $26.60   $26.60  
  Purchase #7 $182.34   3.647%   $188.9899   $188.99   $189.00   $189.00  
  7 purchase totals = $624.32       $669.8961   $669.90   $669.90   $669.90