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 $244.72   12.348%   $274.938   $274.94   $274.95   $274.90  
  Purchase #2 $9.51   4.244%   $9.9136   $9.91   $9.90   $9.90  
  Purchase #3 $84.86   7.580%   $91.2924   $91.29   $91.30   $91.30  
  Purchase #4 $124.60   7.577%   $134.0409   $134.04   $134.05   $134.00  
  Purchase #5 $102.37   10.761%   $113.386   $113.39   $113.40   $113.40  
  Purchase #6 $21.74   13.157%   $24.6003   $24.60   $24.60   $24.60  
  Purchase #7 $168.73   4.649%   $176.5743   $176.57   $176.55   $176.60  
  7 purchase totals = $756.53       $824.7455   $824.74   $824.75   $824.70