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 $100.78   8.610%   $109.4572   $109.46   $109.45   $109.50  
  Purchase #2 $31.72   9.632%   $34.7753   $34.78   $34.80   $34.80  
  Purchase #3 $244.88   13.673%   $278.3624   $278.36   $278.35   $278.40  
  Purchase #4 $44.95   10.343%   $49.5992   $49.60   $49.60   $49.60  
  Purchase #5 $14.98   3.655%   $15.5275   $15.53   $15.55   $15.50  
  Purchase #6 $205.95   5.743%   $217.7777   $217.78   $217.80   $217.80  
  Purchase #7 $221.61   5.486%   $233.7675   $233.77   $233.75   $233.80  
  7 purchase totals = $864.87       $939.2668   $939.28   $939.30   $939.40