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 $23.70   12.421%   $26.6438   $26.64   $26.65   $26.60  
  Purchase #2 $161.44   12.382%   $181.4295   $181.43   $181.45   $181.40  
  Purchase #3 $141.48   4.318%   $147.5891   $147.59   $147.60   $147.60  
  Purchase #4 $32.70   7.226%   $35.0629   $35.06   $35.05   $35.10  
  Purchase #5 $197.28   10.506%   $218.0062   $218.01   $218.00   $218.00  
  Purchase #6 $15.91   13.479%   $18.0545   $18.05   $18.05   $18.10  
  Purchase #7 $11.62   11.352%   $12.9391   $12.94   $12.95   $12.90  
  7 purchase totals = $584.13       $639.7251   $639.72   $639.75   $639.70