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 $27.57   6.415%   $29.3386   $29.34   $29.35   $29.30  
  Purchase #2 $60.74   12.573%   $68.3768   $68.38   $68.40   $68.40  
  Purchase #3 $42.92   8.509%   $46.5721   $46.57   $46.55   $46.60  
  Purchase #4 $221.43   7.792%   $238.6838   $238.68   $238.70   $238.70  
  Purchase #5 $124.82   3.313%   $128.9553   $128.96   $128.95   $129.00  
  Purchase #6 $6.52   9.390%   $7.1322   $7.13   $7.15   $7.10  
  Purchase #7 $115.67   6.489%   $123.1758   $123.18   $123.20   $123.20  
  7 purchase totals = $599.67       $642.2346   $642.24   $642.30   $642.30