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 $236.66   8.592%   $256.9938   $256.99   $257.00   $257.00  
  Purchase #2 $238.67   4.862%   $250.2741   $250.27   $250.25   $250.30  
  Purchase #3 $38.98   6.329%   $41.447   $41.45   $41.45   $41.40  
  Purchase #4 $214.27   13.287%   $242.7401   $242.74   $242.75   $242.70  
  Purchase #5 $65.64   12.513%   $73.8535   $73.85   $73.85   $73.90  
  Purchase #6 $93.37   11.587%   $104.1888   $104.19   $104.20   $104.20  
  Purchase #7 $41.27   7.298%   $44.2819   $44.28   $44.30   $44.30  
  7 purchase totals = $928.86       $1,013.7792   $1,013.77   $1,013.80   $1,013.80