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 $113.34   13.451%   $128.5854   $128.59   $128.60   $128.60  
  Purchase #2 $172.97   11.773%   $193.3338   $193.33   $193.35   $193.30  
  Purchase #3 $242.55   11.286%   $269.9242   $269.92   $269.90   $269.90  
  Purchase #4 $241.48   7.365%   $259.265   $259.27   $259.25   $259.30  
  Purchase #5 $118.37   3.297%   $122.2727   $122.27   $122.25   $122.30  
  Purchase #6 $26.80   9.611%   $29.3757   $29.38   $29.40   $29.40  
  Purchase #7 $242.87   9.163%   $265.1242   $265.12   $265.10   $265.10  
  7 purchase totals = $1,158.38       $1,267.8810   $1,267.88   $1,267.85   $1,267.90