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 $76.64   12.569%   $86.2729   $86.27   $86.25   $86.30  
  Purchase #2 $136.44   9.533%   $149.4468   $149.45   $149.45   $149.40  
  Purchase #3 $89.27   12.681%   $100.5903   $100.59   $100.60   $100.60  
  Purchase #4 $49.82   10.431%   $55.0167   $55.02   $55.00   $55.00  
  Purchase #5 $74.44   13.489%   $84.4812   $84.48   $84.50   $84.50  
  Purchase #6 $149.40   12.873%   $168.6323   $168.63   $168.65   $168.60  
  Purchase #7 $25.41   4.566%   $26.5702   $26.57   $26.55   $26.60  
  7 purchase totals = $601.42       $671.0104   $671.01   $671.00   $671.00