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 $75.25   3.870%   $78.1622   $78.16   $78.15   $78.20  
  Purchase #2 $201.28   8.198%   $217.7809   $217.78   $217.80   $217.80  
  Purchase #3 $236.94   7.405%   $254.4854   $254.49   $254.50   $254.50  
  Purchase #4 $18.81   12.462%   $21.1541   $21.15   $21.15   $21.20  
  Purchase #5 $21.29   3.349%   $22.003   $22.00   $22.00   $22.00  
  Purchase #6 $6.27   11.381%   $6.9836   $6.98   $7.00   $7.00  
  Purchase #7 $42.32   5.596%   $44.6882   $44.69   $44.70   $44.70  
  7 purchase totals = $602.16       $645.2574   $645.25   $645.30   $645.40