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 $76.77   9.115%   $83.7676   $83.77   $83.75   $83.80  
  Purchase #2 $248.89   13.532%   $282.5698   $282.57   $282.55   $282.60  
  Purchase #3 $176.41   9.661%   $193.453   $193.45   $193.45   $193.50  
  Purchase #4 $209.49   12.359%   $235.3809   $235.38   $235.40   $235.40  
  Purchase #5 $152.32   5.866%   $161.2551   $161.26   $161.25   $161.30  
  Purchase #6 $221.77   3.712%   $230.0021   $230.00   $230.00   $230.00  
  Purchase #7 $203.62   4.183%   $212.1374   $212.14   $212.15   $212.10  
  7 purchase totals = $1,289.27       $1,398.5659   $1,398.57   $1,398.55   $1,398.70