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 $246.91   6.825%   $263.7616   $263.76   $263.75   $263.80  
  Purchase #2 $82.51   7.570%   $88.756   $88.76   $88.75   $88.80  
  Purchase #3 $146.63   11.528%   $163.5335   $163.53   $163.55   $163.50  
  Purchase #4 $133.53   3.427%   $138.1061   $138.11   $138.10   $138.10  
  Purchase #5 $7.80   9.609%   $8.5495   $8.55   $8.55   $8.50  
  Purchase #6 $172.91   3.811%   $179.4996   $179.50   $179.50   $179.50  
  Purchase #7 $135.76   13.522%   $154.1175   $154.12   $154.10   $154.10  
  7 purchase totals = $926.05       $996.3238   $996.33   $996.30   $996.30