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 $240.65   13.624%   $273.4362   $273.44   $273.45   $273.40  
  Purchase #2 $24.80   10.839%   $27.4881   $27.49   $27.50   $27.50  
  Purchase #3 $147.29   6.391%   $156.7033   $156.70   $156.70   $156.70  
  Purchase #4 $128.91   11.115%   $143.2383   $143.24   $143.25   $143.20  
  Purchase #5 $127.65   6.789%   $136.3162   $136.32   $136.30   $136.30  
  Purchase #6 $209.84   3.714%   $217.6335   $217.63   $217.65   $217.60  
  Purchase #7 $62.28   4.264%   $64.9356   $64.94   $64.95   $64.90  
  7 purchase totals = $941.42       $1,019.7512   $1,019.76   $1,019.80   $1,019.60