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 $228.82   13.256%   $259.1524   $259.15   $259.15   $259.20  
  Purchase #2 $114.53   4.718%   $119.9335   $119.93   $119.95   $119.90  
  Purchase #3 $233.98   6.176%   $248.4306   $248.43   $248.45   $248.40  
  Purchase #4 $60.66   11.546%   $67.6638   $67.66   $67.65   $67.70  
  Purchase #5 $104.82   6.869%   $112.0201   $112.02   $112.00   $112.00  
  Purchase #6 $37.97   7.689%   $40.8895   $40.89   $40.90   $40.90  
  Purchase #7 $124.64   7.505%   $133.9942   $133.99   $134.00   $134.00  
  7 purchase totals = $905.42       $982.0841   $982.07   $982.10   $982.10