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 $27.90   10.319%   $30.779   $30.78   $30.80   $30.80  
  Purchase #2 $121.57   7.827%   $131.0853   $131.09   $131.10   $131.10  
  Purchase #3 $197.31   8.548%   $214.1761   $214.18   $214.20   $214.20  
  Purchase #4 $235.34   6.788%   $251.3149   $251.31   $251.30   $251.30  
  Purchase #5 $134.85   8.126%   $145.8079   $145.81   $145.80   $145.80  
  Purchase #6 $80.45   5.220%   $84.6495   $84.65   $84.65   $84.60  
  Purchase #7 $49.89   11.680%   $55.7172   $55.72   $55.70   $55.70  
  7 purchase totals = $847.31       $913.5299   $913.54   $913.55   $913.50