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 $13.59   11.164%   $15.1072   $15.11   $15.10   $15.10  
  Purchase #2 $113.43   13.638%   $128.8996   $128.90   $128.90   $128.90  
  Purchase #3 $129.64   9.430%   $141.8651   $141.87   $141.85   $141.90  
  Purchase #4 $217.75   13.499%   $247.1441   $247.14   $247.15   $247.10  
  Purchase #5 $137.33   12.547%   $154.5608   $154.56   $154.55   $154.60  
  Purchase #6 $156.78   9.264%   $171.3041   $171.30   $171.30   $171.30  
  Purchase #7 $123.73   5.375%   $130.3805   $130.38   $130.40   $130.40  
  7 purchase totals = $892.25       $989.2614   $989.26   $989.25   $989.30