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 $13.93   10.566%   $15.4018   $15.40   $15.40   $15.40  
  Purchase #2 $135.51   13.188%   $153.3811   $153.38   $153.40   $153.40  
  Purchase #3 $204.77   7.547%   $220.224   $220.22   $220.20   $220.20  
  Purchase #4 $218.55   13.701%   $248.4935   $248.49   $248.50   $248.50  
  Purchase #5 $154.43   4.537%   $161.4365   $161.44   $161.45   $161.40  
  Purchase #6 $81.93   7.336%   $87.9404   $87.94   $87.95   $87.90  
  Purchase #7 $96.80   6.332%   $102.9294   $102.93   $102.95   $102.90  
  7 purchase totals = $905.92       $989.8067   $989.80   $989.85   $989.70