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 $49.43   11.239%   $54.9854   $54.99   $55.00   $55.00  
  Purchase #2 $86.63   13.256%   $98.1137   $98.11   $98.10   $98.10  
  Purchase #3 $198.73   13.844%   $226.2422   $226.24   $226.25   $226.20  
  Purchase #4 $152.89   10.476%   $168.9068   $168.91   $168.90   $168.90  
  Purchase #5 $13.79   7.672%   $14.848   $14.85   $14.85   $14.80  
  Purchase #6 $34.58   10.243%   $38.122   $38.12   $38.10   $38.10  
  Purchase #7 $204.65   13.701%   $232.6891   $232.69   $232.70   $232.70  
  7 purchase totals = $740.70       $833.9072   $833.91   $833.90   $833.80