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 $249.62   13.753%   $283.9502   $283.95   $283.95   $284.00  
  Purchase #2 $30.33   10.268%   $33.4443   $33.44   $33.45   $33.40  
  Purchase #3 $212.95   10.385%   $235.0649   $235.06   $235.05   $235.10  
  Purchase #4 $177.27   3.486%   $183.4496   $183.45   $183.45   $183.40  
  Purchase #5 $175.36   6.738%   $187.1758   $187.18   $187.20   $187.20  
  Purchase #6 $223.99   11.310%   $249.3233   $249.32   $249.30   $249.30  
  Purchase #7 $138.40   9.816%   $151.9853   $151.99   $152.00   $152.00  
  7 purchase totals = $1,207.92       $1,324.3934   $1,324.39   $1,324.40   $1,324.40