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 $123.64   12.483%   $139.074   $139.07   $139.05   $139.10  
  Purchase #2 $134.63   11.183%   $149.6857   $149.69   $149.70   $149.70  
  Purchase #3 $219.34   13.627%   $249.2295   $249.23   $249.25   $249.20  
  Purchase #4 $205.84   11.760%   $230.0468   $230.05   $230.05   $230.00  
  Purchase #5 $13.98   10.267%   $15.4153   $15.42   $15.40   $15.40  
  Purchase #6 $15.99   3.428%   $16.5381   $16.54   $16.55   $16.50  
  Purchase #7 $226.29   5.601%   $238.9645   $238.96   $238.95   $239.00  
  7 purchase totals = $939.71       $1,038.9539   $1,038.96   $1,038.95   $1,038.90