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 $27.81   7.823%   $29.9856   $29.99   $30.00   $30.00  
  Purchase #2 $66.76   12.352%   $75.0062   $75.01   $75.00   $75.00  
  Purchase #3 $30.35   6.174%   $32.2238   $32.22   $32.20   $32.20  
  Purchase #4 $77.49   4.183%   $80.7314   $80.73   $80.75   $80.70  
  Purchase #5 $7.32   11.389%   $8.1537   $8.15   $8.15   $8.20  
  Purchase #6 $155.67   9.735%   $170.8245   $170.82   $170.80   $170.80  
  Purchase #7 $209.34   4.341%   $218.4274   $218.43   $218.45   $218.40  
  7 purchase totals = $574.74       $615.3526   $615.35   $615.35   $615.30