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 $56.86   5.849%   $60.1857   $60.19   $60.20   $60.20  
  Purchase #2 $28.61   7.266%   $30.6888   $30.69   $30.70   $30.70  
  Purchase #3 $63.96   3.694%   $66.3227   $66.32   $66.30   $66.30  
  Purchase #4 $55.50   9.584%   $60.8191   $60.82   $60.80   $60.80  
  Purchase #5 $184.98   5.338%   $194.8542   $194.85   $194.85   $194.90  
  Purchase #6 $142.78   4.594%   $149.3393   $149.34   $149.35   $149.30  
  Purchase #7 $59.56   3.786%   $61.8149   $61.81   $61.80   $61.80  
  7 purchase totals = $592.25       $624.0247   $624.02   $624.00   $624.00