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 $15.70   7.289%   $16.8444   $16.84   $16.85   $16.80  
  Purchase #2 $47.76   3.469%   $49.4168   $49.42   $49.40   $49.40  
  Purchase #3 $137.27   10.309%   $151.4212   $151.42   $151.40   $151.40  
  Purchase #4 $18.47   10.455%   $20.401   $20.40   $20.40   $20.40  
  Purchase #5 $50.52   9.703%   $55.422   $55.42   $55.40   $55.40  
  Purchase #6 $130.64   8.663%   $141.9573   $141.96   $141.95   $142.00  
  Purchase #7 $89.57   10.314%   $98.8082   $98.81   $98.80   $98.80  
  7 purchase totals = $489.93       $534.2709   $534.27   $534.20   $534.20