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 $189.99   9.776%   $208.5634   $208.56   $208.55   $208.60  
  Purchase #2 $232.71   7.697%   $250.6217   $250.62   $250.60   $250.60  
  Purchase #3 $189.65   7.706%   $204.2644   $204.26   $204.25   $204.30  
  Purchase #4 $249.57   5.734%   $263.8803   $263.88   $263.90   $263.90  
  Purchase #5 $77.82   9.497%   $85.2106   $85.21   $85.20   $85.20  
  Purchase #6 $121.77   11.665%   $135.9745   $135.97   $135.95   $136.00  
  Purchase #7 $205.91   4.159%   $214.4738   $214.47   $214.45   $214.50  
  7 purchase totals = $1,267.42       $1,362.9887   $1,362.97   $1,362.90   $1,363.10