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 $236.88   13.258%   $268.2856   $268.29   $268.30   $268.30  
  Purchase #2 $174.92   6.719%   $186.6729   $186.67   $186.65   $186.70  
  Purchase #3 $154.35   7.130%   $165.3552   $165.36   $165.35   $165.40  
  Purchase #4 $246.57   10.490%   $272.4352   $272.44   $272.45   $272.40  
  Purchase #5 $213.43   12.452%   $240.0063   $240.01   $240.00   $240.00  
  Purchase #6 $135.66   8.530%   $147.2318   $147.23   $147.25   $147.20  
  Purchase #7 $144.25   10.501%   $159.3977   $159.40   $159.40   $159.40  
  7 purchase totals = $1,306.06       $1,439.3847   $1,439.40   $1,439.40   $1,439.40