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 $250.35   7.743%   $269.7346   $269.73   $269.75   $269.70  
  Purchase #2 $83.96   11.556%   $93.6624   $93.66   $93.65   $93.70  
  Purchase #3 $75.96   13.652%   $86.3301   $86.33   $86.35   $86.30  
  Purchase #4 $115.61   12.413%   $129.9607   $129.96   $129.95   $130.00  
  Purchase #5 $8.73   10.733%   $9.667   $9.67   $9.65   $9.70  
  Purchase #6 $145.25   10.461%   $160.4446   $160.44   $160.45   $160.40  
  Purchase #7 $214.34   7.627%   $230.6877   $230.69   $230.70   $230.70  
  7 purchase totals = $894.20       $980.4871   $980.48   $980.50   $980.50