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 $43.51   13.774%   $49.5031   $49.50   $49.50   $49.50  
  Purchase #2 $163.50   7.232%   $175.3243   $175.32   $175.30   $175.30  
  Purchase #3 $67.35   9.298%   $73.6122   $73.61   $73.60   $73.60  
  Purchase #4 $204.29   8.323%   $221.2931   $221.29   $221.30   $221.30  
  Purchase #5 $12.42   6.845%   $13.2701   $13.27   $13.25   $13.30  
  Purchase #6 $96.26   12.427%   $108.2222   $108.22   $108.20   $108.20  
  Purchase #7 $187.90   5.248%   $197.761   $197.76   $197.75   $197.80  
  7 purchase totals = $775.23       $838.9860   $838.97   $838.90   $839.00