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 $246.32   11.113%   $273.6935   $273.69   $273.70   $273.70  
  Purchase #2 $174.80   4.590%   $182.8233   $182.82   $182.80   $182.80  
  Purchase #3 $111.31   11.756%   $124.3956   $124.40   $124.40   $124.40  
  Purchase #4 $35.68   5.210%   $37.5389   $37.54   $37.55   $37.50  
  Purchase #5 $88.63   8.110%   $95.8179   $95.82   $95.80   $95.80  
  Purchase #6 $139.58   4.126%   $145.3391   $145.34   $145.35   $145.30  
  Purchase #7 $183.51   7.795%   $197.8146   $197.81   $197.80   $197.80  
  7 purchase totals = $979.83       $1,057.4229   $1,057.42   $1,057.40   $1,057.30