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 $97.75   3.127%   $100.8066   $100.81   $100.80   $100.80  
  Purchase #2 $138.96   3.792%   $144.2294   $144.23   $144.25   $144.20  
  Purchase #3 $23.59   6.591%   $25.1448   $25.14   $25.15   $25.10  
  Purchase #4 $42.82   10.340%   $47.2476   $47.25   $47.25   $47.20  
  Purchase #5 $21.80   8.778%   $23.7136   $23.71   $23.70   $23.70  
  Purchase #6 $138.50   3.588%   $143.4694   $143.47   $143.45   $143.50  
  Purchase #7 $191.63   3.857%   $199.0212   $199.02   $199.00   $199.00  
  7 purchase totals = $655.05       $683.6326   $683.63   $683.60   $683.50