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 $18.52   13.288%   $20.9809   $20.98   $21.00   $21.00  
  Purchase #2 $76.30   10.180%   $84.0673   $84.07   $84.05   $84.10  
  Purchase #3 $118.71   4.782%   $124.3867   $124.39   $124.40   $124.40  
  Purchase #4 $102.25   13.151%   $115.6969   $115.70   $115.70   $115.70  
  Purchase #5 $128.90   12.515%   $145.0318   $145.03   $145.05   $145.00  
  Purchase #6 $198.39   3.815%   $205.9586   $205.96   $205.95   $206.00  
  Purchase #7 $41.90   11.452%   $46.6984   $46.70   $46.70   $46.70  
  7 purchase totals = $684.97       $742.8206   $742.83   $742.85   $742.90