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 $69.90   13.559%   $79.3777   $79.38   $79.40   $79.40  
  Purchase #2 $19.46   11.442%   $21.6866   $21.69   $21.70   $21.70  
  Purchase #3 $26.91   7.324%   $28.8809   $28.88   $28.90   $28.90  
  Purchase #4 $138.46   10.573%   $153.0994   $153.10   $153.10   $153.10  
  Purchase #5 $193.91   8.652%   $210.6871   $210.69   $210.70   $210.70  
  Purchase #6 $37.60   13.524%   $42.685   $42.69   $42.70   $42.70  
  Purchase #7 $21.54   3.142%   $22.2168   $22.22   $22.20   $22.20  
  7 purchase totals = $507.78       $558.6335   $558.65   $558.70   $558.70