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 $12.69   7.780%   $13.6773   $13.68   $13.70   $13.70  
  Purchase #2 $12.42   12.258%   $13.9424   $13.94   $13.95   $13.90  
  Purchase #3 $36.26   13.390%   $41.1152   $41.12   $41.10   $41.10  
  Purchase #4 $28.41   11.263%   $31.6098   $31.61   $31.60   $31.60  
  Purchase #5 $16.81   13.730%   $19.118   $19.12   $19.10   $19.10  
  Purchase #6 $221.84   8.305%   $240.2638   $240.26   $240.25   $240.30  
  Purchase #7 $227.66   9.866%   $250.1209   $250.12   $250.10   $250.10  
  7 purchase totals = $556.09       $609.8474   $609.85   $609.80   $609.80