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 $19.64   7.118%   $21.038   $21.04   $21.05   $21.00  
  Purchase #2 $202.33   8.643%   $219.8174   $219.82   $219.80   $219.80  
  Purchase #3 $145.36   4.624%   $152.0814   $152.08   $152.10   $152.10  
  Purchase #4 $12.37   3.745%   $12.8333   $12.83   $12.85   $12.80  
  Purchase #5 $249.52   3.137%   $257.3474   $257.35   $257.35   $257.30  
  Purchase #6 $172.68   5.280%   $181.7975   $181.80   $181.80   $181.80  
  Purchase #7 $35.93   5.858%   $38.0348   $38.03   $38.05   $38.00  
  7 purchase totals = $837.83       $882.9498   $882.95   $883.00   $882.80