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 $128.51   8.156%   $138.9913   $138.99   $139.00   $139.00  
  Purchase #2 $146.80   8.147%   $158.7598   $158.76   $158.75   $158.80  
  Purchase #3 $132.36   10.176%   $145.829   $145.83   $145.85   $145.80  
  Purchase #4 $10.45   12.144%   $11.719   $11.72   $11.70   $11.70  
  Purchase #5 $128.68   11.287%   $143.2041   $143.20   $143.20   $143.20  
  Purchase #6 $180.86   12.658%   $203.7533   $203.75   $203.75   $203.80  
  Purchase #7 $90.48   13.489%   $102.6848   $102.68   $102.70   $102.70  
  7 purchase totals = $818.14       $904.9413   $904.93   $904.95   $905.00