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 $59.99   10.338%   $66.1918   $66.19   $66.20   $66.20  
  Purchase #2 $202.93   8.629%   $220.4408   $220.44   $220.45   $220.40  
  Purchase #3 $123.73   13.387%   $140.2937   $140.29   $140.30   $140.30  
  Purchase #4 $42.46   10.731%   $47.0164   $47.02   $47.00   $47.00  
  Purchase #5 $81.73   10.376%   $90.2103   $90.21   $90.20   $90.20  
  Purchase #6 $55.99   5.481%   $59.0588   $59.06   $59.05   $59.10  
  Purchase #7 $134.70   13.433%   $152.7943   $152.79   $152.80   $152.80  
  7 purchase totals = $701.53       $776.0061   $776.00   $776.00   $776.00