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 $54.57   6.717%   $58.2355   $58.24   $58.25   $58.20  
  Purchase #2 $52.40   5.506%   $55.2851   $55.29   $55.30   $55.30  
  Purchase #3 $157.68   10.622%   $174.4288   $174.43   $174.45   $174.40  
  Purchase #4 $64.29   5.776%   $68.0034   $68.00   $68.00   $68.00  
  Purchase #5 $11.61   7.115%   $12.4361   $12.44   $12.45   $12.40  
  Purchase #6 $226.81   4.452%   $236.9076   $236.91   $236.90   $236.90  
  Purchase #7 $23.72   6.571%   $25.2786   $25.28   $25.30   $25.30  
  7 purchase totals = $591.08       $630.5751   $630.59   $630.65   $630.50