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 $204.87   12.564%   $230.6099   $230.61   $230.60   $230.60  
  Purchase #2 $48.91   5.108%   $51.4083   $51.41   $51.40   $51.40  
  Purchase #3 $192.29   3.763%   $199.5259   $199.53   $199.55   $199.50  
  Purchase #4 $133.67   7.563%   $143.7795   $143.78   $143.80   $143.80  
  Purchase #5 $134.35   11.857%   $150.2799   $150.28   $150.30   $150.30  
  Purchase #6 $70.75   3.653%   $73.3345   $73.33   $73.35   $73.30  
  Purchase #7 $5.26   13.839%   $5.9879   $5.99   $6.00   $6.00  
  7 purchase totals = $790.10       $854.9259   $854.93   $855.00   $854.90