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 $122.50   12.712%   $138.0722   $138.07   $138.05   $138.10  
  Purchase #2 $177.86   3.488%   $184.0638   $184.06   $184.05   $184.10  
  Purchase #3 $134.35   13.831%   $152.9319   $152.93   $152.95   $152.90  
  Purchase #4 $11.86   9.530%   $12.9903   $12.99   $13.00   $13.00  
  Purchase #5 $234.39   6.216%   $248.9597   $248.96   $248.95   $249.00  
  Purchase #6 $154.39   12.569%   $173.7953   $173.80   $173.80   $173.80  
  Purchase #7 $164.98   10.368%   $182.0851   $182.09   $182.10   $182.10  
  7 purchase totals = $1,000.33       $1,092.8983   $1,092.90   $1,092.90   $1,093.00