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 $107.77   12.150%   $120.8641   $120.86   $120.85   $120.90  
  Purchase #2 $212.76   7.382%   $228.4659   $228.47   $228.45   $228.50  
  Purchase #3 $176.44   7.580%   $189.8142   $189.81   $189.80   $189.80  
  Purchase #4 $11.35   13.406%   $12.8716   $12.87   $12.85   $12.90  
  Purchase #5 $108.38   7.737%   $116.7654   $116.77   $116.75   $116.80  
  Purchase #6 $249.86   6.736%   $266.6906   $266.69   $266.70   $266.70  
  Purchase #7 $208.80   9.704%   $229.062   $229.06   $229.05   $229.10  
  7 purchase totals = $1,075.36       $1,164.5338   $1,164.53   $1,164.45   $1,164.70