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 $184.92   12.211%   $207.5006   $207.50   $207.50   $207.50  
  Purchase #2 $140.77   4.173%   $146.6443   $146.64   $146.65   $146.60  
  Purchase #3 $77.99   6.751%   $83.2551   $83.26   $83.25   $83.30  
  Purchase #4 $165.25   4.213%   $172.212   $172.21   $172.20   $172.20  
  Purchase #5 $239.44   11.440%   $266.8319   $266.83   $266.85   $266.80  
  Purchase #6 $137.66   6.660%   $146.8282   $146.83   $146.85   $146.80  
  Purchase #7 $15.49   6.812%   $16.5452   $16.55   $16.55   $16.50  
  7 purchase totals = $961.52       $1,039.8173   $1,039.82   $1,039.85   $1,039.70