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 $44.41   8.763%   $48.3016   $48.30   $48.30   $48.30  
  Purchase #2 $80.58   4.815%   $84.4599   $84.46   $84.45   $84.50  
  Purchase #3 $61.30   8.312%   $66.3953   $66.40   $66.40   $66.40  
  Purchase #4 $101.96   4.664%   $106.7154   $106.72   $106.70   $106.70  
  Purchase #5 $191.41   12.129%   $214.6261   $214.63   $214.65   $214.60  
  Purchase #6 $44.55   6.631%   $47.5041   $47.50   $47.50   $47.50  
  Purchase #7 $244.54   6.651%   $260.8044   $260.80   $260.80   $260.80  
  7 purchase totals = $768.75       $828.8068   $828.81   $828.80   $828.80