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 $8.66   4.436%   $9.0442   $9.04   $9.05   $9.00  
  Purchase #2 $73.67   8.718%   $80.0926   $80.09   $80.10   $80.10  
  Purchase #3 $7.37   3.259%   $7.6102   $7.61   $7.60   $7.60  
  Purchase #4 $218.54   13.686%   $248.4494   $248.45   $248.45   $248.40  
  Purchase #5 $167.75   12.541%   $188.7875   $188.79   $188.80   $188.80  
  Purchase #6 $33.57   10.650%   $37.1452   $37.15   $37.15   $37.10  
  Purchase #7 $96.51   12.682%   $108.7494   $108.75   $108.75   $108.70  
  7 purchase totals = $606.07       $679.8785   $679.88   $679.90   $679.70