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   4.207%   $21.2061   $21.21   $21.20   $21.20  
  Purchase #2 $120.27   7.736%   $129.5741   $129.57   $129.55   $129.60  
  Purchase #3 $64.47   13.209%   $72.9858   $72.99   $73.00   $73.00  
  Purchase #4 $233.88   8.407%   $253.5423   $253.54   $253.55   $253.50  
  Purchase #5 $192.80   13.723%   $219.2579   $219.26   $219.25   $219.30  
  Purchase #6 $6.61   7.151%   $7.0827   $7.08   $7.10   $7.10  
  Purchase #7 $193.68   4.486%   $202.3685   $202.37   $202.35   $202.40  
  7 purchase totals = $832.06       $906.0174   $906.02   $906.00   $906.10