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 $122.63   5.286%   $129.1122   $129.11   $129.10   $129.10  
  Purchase #2 $172.91   3.862%   $179.5878   $179.59   $179.60   $179.60  
  Purchase #3 $131.63   10.814%   $145.8645   $145.86   $145.85   $145.90  
  Purchase #4 $131.30   12.790%   $148.0933   $148.09   $148.10   $148.10  
  Purchase #5 $12.44   5.184%   $13.0849   $13.08   $13.10   $13.10  
  Purchase #6 $172.31   11.651%   $192.3858   $192.39   $192.40   $192.40  
  Purchase #7 $153.38   13.223%   $173.6614   $173.66   $173.65   $173.70  
  7 purchase totals = $896.60       $981.7899   $981.78   $981.80   $981.90