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 $120.80   9.330%   $132.0706   $132.07   $132.05   $132.10  
  Purchase #2 $6.61   12.743%   $7.4523   $7.45   $7.45   $7.50  
  Purchase #3 $132.97   5.394%   $140.1424   $140.14   $140.15   $140.10  
  Purchase #4 $16.74   12.840%   $18.8894   $18.89   $18.90   $18.90  
  Purchase #5 $10.50   6.854%   $11.2197   $11.22   $11.20   $11.20  
  Purchase #6 $199.82   9.589%   $218.9807   $218.98   $219.00   $219.00  
  Purchase #7 $43.30   10.458%   $47.8283   $47.83   $47.85   $47.80  
  7 purchase totals = $530.74       $576.5834   $576.58   $576.60   $576.60