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 $28.41   8.346%   $30.7811   $30.78   $30.80   $30.80  
  Purchase #2 $128.28   7.671%   $138.1204   $138.12   $138.10   $138.10  
  Purchase #3 $17.47   7.199%   $18.7277   $18.73   $18.75   $18.70  
  Purchase #4 $81.93   10.184%   $90.2738   $90.27   $90.25   $90.30  
  Purchase #5 $5.87   6.566%   $6.2554   $6.26   $6.25   $6.30  
  Purchase #6 $103.29   12.348%   $116.0442   $116.04   $116.05   $116.00  
  Purchase #7 $220.94   7.402%   $237.294   $237.29   $237.30   $237.30  
  7 purchase totals = $586.19       $637.4966   $637.49   $637.50   $637.50