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 $21.55   7.509%   $23.1682   $23.17   $23.15   $23.20  
  Purchase #2 $128.58   11.304%   $143.1147   $143.11   $143.10   $143.10  
  Purchase #3 $22.35   13.147%   $25.2884   $25.29   $25.30   $25.30  
  Purchase #4 $174.38   6.786%   $186.2134   $186.21   $186.20   $186.20  
  Purchase #5 $13.65   3.184%   $14.0846   $14.08   $14.10   $14.10  
  Purchase #6 $214.98   13.456%   $243.9077   $243.91   $243.90   $243.90  
  Purchase #7 $31.84   8.161%   $34.4385   $34.44   $34.45   $34.40  
  7 purchase totals = $607.33       $670.2155   $670.21   $670.20   $670.20