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 $211.85   13.143%   $239.6934   $239.69   $239.70   $239.70  
  Purchase #2 $114.98   4.718%   $120.4048   $120.40   $120.40   $120.40  
  Purchase #3 $134.51   6.784%   $143.6352   $143.64   $143.65   $143.60  
  Purchase #4 $88.70   10.125%   $97.6809   $97.68   $97.70   $97.70  
  Purchase #5 $164.35   11.625%   $183.4557   $183.46   $183.45   $183.50  
  Purchase #6 $61.74   9.119%   $67.3701   $67.37   $67.35   $67.40  
  Purchase #7 $122.61   5.757%   $129.6687   $129.67   $129.65   $129.70  
  7 purchase totals = $898.74       $981.9088   $981.91   $981.90   $982.00