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 $20.35   4.323%   $21.2297   $21.23   $21.25   $21.20  
  Purchase #2 $37.51   9.571%   $41.1001   $41.10   $41.10   $41.10  
  Purchase #3 $227.33   6.565%   $242.2542   $242.25   $242.25   $242.30  
  Purchase #4 $189.68   7.687%   $204.2607   $204.26   $204.25   $204.30  
  Purchase #5 $115.56   4.665%   $120.9509   $120.95   $120.95   $121.00  
  Purchase #6 $37.34   11.760%   $41.7312   $41.73   $41.75   $41.70  
  Purchase #7 $8.91   11.276%   $9.9147   $9.91   $9.90   $9.90  
  7 purchase totals = $636.68       $681.4415   $681.43   $681.45   $681.50