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 $249.87   13.580%   $283.8023   $283.80   $283.80   $283.80  
  Purchase #2 $146.98   10.184%   $161.9484   $161.95   $161.95   $161.90  
  Purchase #3 $84.48   13.798%   $96.1366   $96.14   $96.15   $96.10  
  Purchase #4 $111.42   7.545%   $119.8266   $119.83   $119.85   $119.80  
  Purchase #5 $227.32   12.278%   $255.2303   $255.23   $255.25   $255.20  
  Purchase #6 $206.49   11.665%   $230.5771   $230.58   $230.60   $230.60  
  Purchase #7 $202.85   9.545%   $222.212   $222.21   $222.20   $222.20  
  7 purchase totals = $1,229.41       $1,369.7333   $1,369.74   $1,369.80   $1,369.60