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 $63.74   10.216%   $70.2517   $70.25   $70.25   $70.30  
  Purchase #2 $250.66   6.252%   $266.3313   $266.33   $266.35   $266.30  
  Purchase #3 $46.89   4.599%   $49.0465   $49.05   $49.05   $49.00  
  Purchase #4 $86.76   5.560%   $91.5839   $91.58   $91.60   $91.60  
  Purchase #5 $138.34   4.837%   $145.0315   $145.03   $145.05   $145.00  
  Purchase #6 $173.82   4.616%   $181.8435   $181.84   $181.85   $181.80  
  Purchase #7 $34.34   13.133%   $38.8499   $38.85   $38.85   $38.80  
  7 purchase totals = $794.55       $842.9383   $842.93   $843.00   $842.80