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 $74.64   5.788%   $78.9602   $78.96   $78.95   $79.00  
  Purchase #2 $144.66   8.513%   $156.9749   $156.97   $156.95   $157.00  
  Purchase #3 $52.43   6.447%   $55.8102   $55.81   $55.80   $55.80  
  Purchase #4 $167.26   12.107%   $187.5102   $187.51   $187.50   $187.50  
  Purchase #5 $76.89   9.870%   $84.479   $84.48   $84.50   $84.50  
  Purchase #6 $27.55   11.386%   $30.6868   $30.69   $30.70   $30.70  
  Purchase #7 $88.35   11.476%   $98.489   $98.49   $98.50   $98.50  
  7 purchase totals = $631.78       $692.9103   $692.91   $692.90   $693.00