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 $147.46   8.437%   $159.9012   $159.90   $159.90   $159.90  
  Purchase #2 $230.98   12.378%   $259.5707   $259.57   $259.55   $259.60  
  Purchase #3 $17.80   9.843%   $19.5521   $19.55   $19.55   $19.60  
  Purchase #4 $9.52   5.366%   $10.0308   $10.03   $10.05   $10.00  
  Purchase #5 $75.50   11.108%   $83.8865   $83.89   $83.90   $83.90  
  Purchase #6 $97.57   3.676%   $101.1567   $101.16   $101.15   $101.20  
  Purchase #7 $89.61   7.503%   $96.3334   $96.33   $96.35   $96.30  
  7 purchase totals = $668.44       $730.4314   $730.43   $730.45   $730.50