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 $129.91   8.854%   $141.4122   $141.41   $141.40   $141.40  
  Purchase #2 $90.37   10.415%   $99.782   $99.78   $99.80   $99.80  
  Purchase #3 $130.37   5.653%   $137.7398   $137.74   $137.75   $137.70  
  Purchase #4 $5.84   10.179%   $6.4345   $6.43   $6.45   $6.40  
  Purchase #5 $41.86   5.689%   $44.2414   $44.24   $44.25   $44.20  
  Purchase #6 $12.68   7.283%   $13.6035   $13.60   $13.60   $13.60  
  Purchase #7 $139.83   10.402%   $154.3751   $154.38   $154.40   $154.40  
  7 purchase totals = $550.86       $597.5885   $597.58   $597.65   $597.50