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 $241.92   13.127%   $273.6768   $273.68   $273.70   $273.70  
  Purchase #2 $74.37   13.860%   $84.6777   $84.68   $84.70   $84.70  
  Purchase #3 $52.38   7.186%   $56.144   $56.14   $56.15   $56.10  
  Purchase #4 $244.50   13.187%   $276.7422   $276.74   $276.75   $276.70  
  Purchase #5 $22.84   9.680%   $25.0509   $25.05   $25.05   $25.10  
  Purchase #6 $112.61   11.685%   $125.7685   $125.77   $125.75   $125.80  
  Purchase #7 $142.61   8.353%   $154.5222   $154.52   $154.50   $154.50  
  7 purchase totals = $891.23       $996.5823   $996.58   $996.60   $996.60