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 $19.50   7.363%   $20.9358   $20.94   $20.95   $20.90  
  Purchase #2 $95.65   5.132%   $100.5588   $100.56   $100.55   $100.60  
  Purchase #3 $43.89   13.705%   $49.9051   $49.91   $49.90   $49.90  
  Purchase #4 $215.86   6.206%   $229.2563   $229.26   $229.25   $229.30  
  Purchase #5 $47.74   3.585%   $49.4515   $49.45   $49.45   $49.50  
  Purchase #6 $30.66   12.122%   $34.3766   $34.38   $34.40   $34.40  
  Purchase #7 $180.77   7.151%   $193.6969   $193.70   $193.70   $193.70  
  7 purchase totals = $634.07       $678.1810   $678.20   $678.20   $678.30