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 $36.69   6.295%   $38.9996   $39.00   $39.00   $39.00  
  Purchase #2 $35.70   10.477%   $39.4403   $39.44   $39.45   $39.40  
  Purchase #3 $76.72   13.789%   $87.2989   $87.30   $87.30   $87.30  
  Purchase #4 $23.50   6.476%   $25.0219   $25.02   $25.00   $25.00  
  Purchase #5 $81.98   8.302%   $88.786   $88.79   $88.80   $88.80  
  Purchase #6 $125.44   11.517%   $139.8869   $139.89   $139.90   $139.90  
  Purchase #7 $201.96   4.855%   $211.7652   $211.77   $211.75   $211.80  
  7 purchase totals = $581.99       $631.1988   $631.21   $631.20   $631.20