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 $225.69   10.561%   $249.5251   $249.53   $249.55   $249.50  
  Purchase #2 $223.34   11.519%   $249.0665   $249.07   $249.05   $249.10  
  Purchase #3 $14.75   6.378%   $15.6908   $15.69   $15.70   $15.70  
  Purchase #4 $228.37   6.872%   $244.0636   $244.06   $244.05   $244.10  
  Purchase #5 $87.81   12.171%   $98.4974   $98.50   $98.50   $98.50  
  Purchase #6 $102.96   11.495%   $114.7953   $114.80   $114.80   $114.80  
  Purchase #7 $181.89   13.130%   $205.7722   $205.77   $205.75   $205.80  
  7 purchase totals = $1,064.81       $1,177.4109   $1,177.42   $1,177.40   $1,177.50