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 $238.84   6.771%   $255.0119   $255.01   $255.00   $255.00  
  Purchase #2 $92.60   12.754%   $104.4102   $104.41   $104.40   $104.40  
  Purchase #3 $26.27   13.843%   $29.9066   $29.91   $29.90   $29.90  
  Purchase #4 $9.65   7.212%   $10.346   $10.35   $10.35   $10.30  
  Purchase #5 $64.66   5.253%   $68.0566   $68.06   $68.05   $68.10  
  Purchase #6 $9.30   8.539%   $10.0941   $10.09   $10.10   $10.10  
  Purchase #7 $205.28   8.731%   $223.203   $223.20   $223.20   $223.20  
  7 purchase totals = $646.60       $701.0284   $701.03   $701.00   $701.00