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 $55.67   10.759%   $61.6595   $61.66   $61.65   $61.70  
  Purchase #2 $153.55   4.625%   $160.6517   $160.65   $160.65   $160.70  
  Purchase #3 $243.96   13.798%   $277.6216   $277.62   $277.60   $277.60  
  Purchase #4 $136.48   4.564%   $142.7089   $142.71   $142.70   $142.70  
  Purchase #5 $26.45   4.319%   $27.5924   $27.59   $27.60   $27.60  
  Purchase #6 $114.71   3.642%   $118.8877   $118.89   $118.90   $118.90  
  Purchase #7 $64.27   8.297%   $69.6025   $69.60   $69.60   $69.60  
  7 purchase totals = $795.09       $858.7243   $858.72   $858.70   $858.80