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 $249.41   11.187%   $277.3115   $277.31   $277.30   $277.30  
  Purchase #2 $250.61   7.353%   $269.0374   $269.04   $269.05   $269.00  
  Purchase #3 $219.96   11.712%   $245.7217   $245.72   $245.70   $245.70  
  Purchase #4 $244.31   9.526%   $267.583   $267.58   $267.60   $267.60  
  Purchase #5 $7.80   3.489%   $8.0721   $8.07   $8.05   $8.10  
  Purchase #6 $150.69   5.302%   $158.6796   $158.68   $158.70   $158.70  
  Purchase #7 $43.31   10.303%   $47.7722   $47.77   $47.75   $47.80  
  7 purchase totals = $1,166.09       $1,274.1775   $1,274.17   $1,274.15   $1,274.20