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 $120.77   9.366%   $132.0813   $132.08   $132.10   $132.10  
  Purchase #2 $36.46   13.737%   $41.4685   $41.47   $41.45   $41.50  
  Purchase #3 $93.31   13.151%   $105.5812   $105.58   $105.60   $105.60  
  Purchase #4 $246.95   13.381%   $279.9944   $279.99   $280.00   $280.00  
  Purchase #5 $133.80   9.725%   $146.8121   $146.81   $146.80   $146.80  
  Purchase #6 $68.43   11.763%   $76.4794   $76.48   $76.50   $76.50  
  Purchase #7 $56.46   4.200%   $58.8313   $58.83   $58.85   $58.80  
  7 purchase totals = $756.18       $841.2482   $841.24   $841.30   $841.30