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 $241.97   13.115%   $273.7044   $273.70   $273.70   $273.70  
  Purchase #2 $114.36   11.174%   $127.1386   $127.14   $127.15   $127.10  
  Purchase #3 $71.70   5.744%   $75.8184   $75.82   $75.80   $75.80  
  Purchase #4 $229.86   13.832%   $261.6542   $261.65   $261.65   $261.70  
  Purchase #5 $199.48   12.832%   $225.0773   $225.08   $225.10   $225.10  
  Purchase #6 $239.78   12.542%   $269.8532   $269.85   $269.85   $269.90  
  Purchase #7 $11.90   12.130%   $13.3435   $13.34   $13.35   $13.30  
  7 purchase totals = $1,109.05       $1,246.5896   $1,246.58   $1,246.60   $1,246.60