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 $24.57   11.699%   $27.4444   $27.44   $27.45   $27.40  
  Purchase #2 $128.96   13.456%   $146.3129   $146.31   $146.30   $146.30  
  Purchase #3 $118.67   13.378%   $134.5457   $134.55   $134.55   $134.50  
  Purchase #4 $168.95   8.372%   $183.0945   $183.09   $183.10   $183.10  
  Purchase #5 $92.79   11.604%   $103.5574   $103.56   $103.55   $103.60  
  Purchase #6 $71.91   5.810%   $76.088   $76.09   $76.10   $76.10  
  Purchase #7 $45.57   13.401%   $51.6768   $51.68   $51.70   $51.70  
  7 purchase totals = $651.42       $722.7197   $722.72   $722.75   $722.70