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 $135.97   6.154%   $144.3376   $144.34   $144.35   $144.30  
  Purchase #2 $44.26   7.480%   $47.5706   $47.57   $47.55   $47.60  
  Purchase #3 $167.34   11.421%   $186.4519   $186.45   $186.45   $186.50  
  Purchase #4 $198.96   7.504%   $213.89   $213.89   $213.90   $213.90  
  Purchase #5 $82.31   8.421%   $89.2413   $89.24   $89.25   $89.20  
  Purchase #6 $89.91   8.617%   $97.6575   $97.66   $97.65   $97.70  
  Purchase #7 $26.67   9.361%   $29.1666   $29.17   $29.15   $29.20  
  7 purchase totals = $745.42       $808.3155   $808.32   $808.30   $808.40