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 $248.74   12.376%   $279.5241   $279.52   $279.50   $279.50  
  Purchase #2 $204.72   3.394%   $211.6682   $211.67   $211.65   $211.70  
  Purchase #3 $58.41   5.198%   $61.4462   $61.45   $61.45   $61.40  
  Purchase #4 $226.66   10.166%   $249.7023   $249.70   $249.70   $249.70  
  Purchase #5 $107.70   6.374%   $114.5648   $114.56   $114.55   $114.60  
  Purchase #6 $92.42   10.419%   $102.0492   $102.05   $102.05   $102.00  
  Purchase #7 $98.28   9.436%   $107.5537   $107.55   $107.55   $107.60  
  7 purchase totals = $1,036.93       $1,126.5085   $1,126.50   $1,126.45   $1,126.50