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 $9.40   10.474%   $10.3846   $10.38   $10.40   $10.40  
  Purchase #2 $16.87   7.512%   $18.1373   $18.14   $18.15   $18.10  
  Purchase #3 $168.78   6.487%   $179.7288   $179.73   $179.75   $179.70  
  Purchase #4 $135.45   4.115%   $141.0238   $141.02   $141.00   $141.00  
  Purchase #5 $35.92   10.191%   $39.5806   $39.58   $39.60   $39.60  
  Purchase #6 $191.34   3.563%   $198.1574   $198.16   $198.15   $198.20  
  Purchase #7 $134.61   5.259%   $141.6891   $141.69   $141.70   $141.70  
  7 purchase totals = $692.37       $728.7016   $728.70   $728.75   $728.70