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 $151.38   11.466%   $168.7372   $168.74   $168.75   $168.70  
  Purchase #2 $181.27   10.402%   $200.1257   $200.13   $200.15   $200.10  
  Purchase #3 $70.35   4.428%   $73.4651   $73.47   $73.45   $73.50  
  Purchase #4 $7.85   13.591%   $8.9169   $8.92   $8.90   $8.90  
  Purchase #5 $17.39   12.564%   $19.5749   $19.57   $19.55   $19.60  
  Purchase #6 $112.92   13.543%   $128.2128   $128.21   $128.20   $128.20  
  Purchase #7 $42.30   4.706%   $44.2906   $44.29   $44.30   $44.30  
  7 purchase totals = $583.46       $643.3232   $643.33   $643.30   $643.30