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 $13.65   10.773%   $15.1205   $15.12   $15.10   $15.10  
  Purchase #2 $173.97   11.195%   $193.4459   $193.45   $193.45   $193.40  
  Purchase #3 $202.95   8.153%   $219.4965   $219.50   $219.50   $219.50  
  Purchase #4 $106.66   10.412%   $117.7654   $117.77   $117.75   $117.80  
  Purchase #5 $57.43   10.118%   $63.2408   $63.24   $63.25   $63.20  
  Purchase #6 $153.81   10.565%   $170.06   $170.06   $170.05   $170.10  
  Purchase #7 $15.35   6.644%   $16.3699   $16.37   $16.35   $16.40  
  7 purchase totals = $723.82       $795.4990   $795.51   $795.45   $795.50