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 $177.68   6.815%   $189.7889   $189.79   $189.80   $189.80  
  Purchase #2 $202.59   8.804%   $220.426   $220.43   $220.45   $220.40  
  Purchase #3 $250.86   10.824%   $278.0131   $278.01   $278.00   $278.00  
  Purchase #4 $43.81   7.593%   $47.1365   $47.14   $47.15   $47.10  
  Purchase #5 $81.25   10.868%   $90.0803   $90.08   $90.10   $90.10  
  Purchase #6 $154.95   13.197%   $175.3988   $175.40   $175.40   $175.40  
  Purchase #7 $91.56   7.162%   $98.1175   $98.12   $98.10   $98.10  
  7 purchase totals = $1,002.70       $1,098.9611   $1,098.97   $1,099.00   $1,098.90