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 $19.38   9.195%   $21.162   $21.16   $21.15   $21.20  
  Purchase #2 $100.50   4.679%   $105.2024   $105.20   $105.20   $105.20  
  Purchase #3 $166.83   5.609%   $176.1875   $176.19   $176.20   $176.20  
  Purchase #4 $9.86   8.397%   $10.6879   $10.69   $10.70   $10.70  
  Purchase #5 $230.30   9.149%   $251.3701   $251.37   $251.35   $251.40  
  Purchase #6 $147.92   7.247%   $158.6398   $158.64   $158.65   $158.60  
  Purchase #7 $120.30   12.290%   $135.0849   $135.08   $135.10   $135.10  
  7 purchase totals = $795.09       $858.3346   $858.33   $858.35   $858.40