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 $5.85   13.789%   $6.6567   $6.66   $6.65   $6.70  
  Purchase #2 $9.46   6.666%   $10.0906   $10.09   $10.10   $10.10  
  Purchase #3 $124.60   6.195%   $132.319   $132.32   $132.30   $132.30  
  Purchase #4 $134.90   13.476%   $153.0791   $153.08   $153.10   $153.10  
  Purchase #5 $20.89   7.317%   $22.4185   $22.42   $22.40   $22.40  
  Purchase #6 $76.28   3.825%   $79.1977   $79.20   $79.20   $79.20  
  Purchase #7 $131.85   7.560%   $141.8179   $141.82   $141.80   $141.80  
  7 purchase totals = $503.83       $545.5795   $545.59   $545.55   $545.60