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 $190.61   12.412%   $214.2685   $214.27   $214.25   $214.30  
  Purchase #2 $126.95   4.711%   $132.9306   $132.93   $132.95   $132.90  
  Purchase #3 $49.86   3.312%   $51.5114   $51.51   $51.50   $51.50  
  Purchase #4 $168.60   8.611%   $183.1181   $183.12   $183.10   $183.10  
  Purchase #5 $55.85   9.445%   $61.125   $61.13   $61.15   $61.10  
  Purchase #6 $183.90   10.403%   $203.0311   $203.03   $203.05   $203.00  
  Purchase #7 $138.92   6.261%   $147.6178   $147.62   $147.60   $147.60  
  7 purchase totals = $914.69       $993.6025   $993.61   $993.60   $993.50