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 $12.51   11.205%   $13.9117   $13.91   $13.90   $13.90  
  Purchase #2 $138.45   8.181%   $149.7766   $149.78   $149.80   $149.80  
  Purchase #3 $7.70   8.623%   $8.364   $8.36   $8.35   $8.40  
  Purchase #4 $166.75   13.745%   $189.6698   $189.67   $189.65   $189.70  
  Purchase #5 $74.74   6.611%   $79.6811   $79.68   $79.70   $79.70  
  Purchase #6 $138.50   3.564%   $143.4361   $143.44   $143.45   $143.40  
  Purchase #7 $100.31   5.630%   $105.9575   $105.96   $105.95   $106.00  
  7 purchase totals = $638.96       $690.7968   $690.80   $690.80   $690.90