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 $16.53   8.734%   $17.9737   $17.97   $17.95   $18.00  
  Purchase #2 $59.80   3.405%   $61.8362   $61.84   $61.85   $61.80  
  Purchase #3 $155.29   12.414%   $174.5677   $174.57   $174.55   $174.60  
  Purchase #4 $6.31   11.764%   $7.0523   $7.05   $7.05   $7.10  
  Purchase #5 $11.32   13.355%   $12.8318   $12.83   $12.85   $12.80  
  Purchase #6 $154.44   13.111%   $174.6886   $174.69   $174.70   $174.70  
  Purchase #7 $124.99   9.804%   $137.244   $137.24   $137.25   $137.20  
  7 purchase totals = $528.68       $586.1943   $586.19   $586.20   $586.20