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 $18.60   7.538%   $20.0021   $20.00   $20.00   $20.00  
  Purchase #2 $71.40   12.341%   $80.2115   $80.21   $80.20   $80.20  
  Purchase #3 $131.58   3.728%   $136.4853   $136.49   $136.50   $136.50  
  Purchase #4 $231.34   6.257%   $245.8149   $245.81   $245.80   $245.80  
  Purchase #5 $241.31   8.202%   $261.1022   $261.10   $261.10   $261.10  
  Purchase #6 $107.58   9.207%   $117.4849   $117.48   $117.50   $117.50  
  Purchase #7 $250.89   9.779%   $275.4245   $275.42   $275.40   $275.40  
  7 purchase totals = $1,052.70       $1,136.5254   $1,136.51   $1,136.50   $1,136.50