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 $5.91   11.670%   $6.5997   $6.60   $6.60   $6.60  
  Purchase #2 $6.29   11.813%   $7.033   $7.03   $7.05   $7.00  
  Purchase #3 $121.48   13.150%   $137.4546   $137.45   $137.45   $137.50  
  Purchase #4 $234.30   7.496%   $251.8631   $251.86   $251.85   $251.90  
  Purchase #5 $33.42   9.865%   $36.7169   $36.72   $36.70   $36.70  
  Purchase #6 $209.71   10.804%   $232.3671   $232.37   $232.35   $232.40  
  Purchase #7 $20.77   3.359%   $21.4677   $21.47   $21.45   $21.50  
  7 purchase totals = $631.88       $693.5021   $693.50   $693.45   $693.60