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 $62.67   12.153%   $70.2863   $70.29   $70.30   $70.30  
  Purchase #2 $83.96   9.155%   $91.6465   $91.65   $91.65   $91.60  
  Purchase #3 $72.45   3.401%   $74.914   $74.91   $74.90   $74.90  
  Purchase #4 $110.88   6.708%   $118.3178   $118.32   $118.30   $118.30  
  Purchase #5 $203.62   11.513%   $227.0628   $227.06   $227.05   $227.10  
  Purchase #6 $7.83   7.679%   $8.4313   $8.43   $8.45   $8.40  
  Purchase #7 $53.40   7.735%   $57.5305   $57.53   $57.55   $57.50  
  7 purchase totals = $594.81       $648.1892   $648.19   $648.20   $648.10