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 $124.29   11.853%   $139.0221   $139.02   $139.00   $139.00  
  Purchase #2 $238.92   4.658%   $250.0489   $250.05   $250.05   $250.00  
  Purchase #3 $240.34   13.585%   $272.9902   $272.99   $273.00   $273.00  
  Purchase #4 $27.62   13.758%   $31.42   $31.42   $31.40   $31.40  
  Purchase #5 $149.65   11.511%   $166.8762   $166.88   $166.90   $166.90  
  Purchase #6 $232.34   4.649%   $243.1415   $243.14   $243.15   $243.10  
  Purchase #7 $106.46   4.205%   $110.9366   $110.94   $110.95   $110.90  
  7 purchase totals = $1,119.62       $1,214.4355   $1,214.44   $1,214.45   $1,214.30