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 $244.28   8.715%   $265.569   $265.57   $265.55   $265.60  
  Purchase #2 $64.28   11.169%   $71.4594   $71.46   $71.45   $71.50  
  Purchase #3 $111.27   8.201%   $120.3953   $120.40   $120.40   $120.40  
  Purchase #4 $238.62   4.235%   $248.7256   $248.73   $248.75   $248.70  
  Purchase #5 $181.64   11.687%   $202.8683   $202.87   $202.85   $202.90  
  Purchase #6 $10.69   4.476%   $11.1685   $11.17   $11.15   $11.20  
  Purchase #7 $92.44   8.683%   $100.4666   $100.47   $100.45   $100.50  
  7 purchase totals = $943.22       $1,020.6527   $1,020.67   $1,020.60   $1,020.80