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 $234.57   12.231%   $263.2603   $263.26   $263.25   $263.30  
  Purchase #2 $86.83   8.523%   $94.2305   $94.23   $94.25   $94.20  
  Purchase #3 $126.47   5.394%   $133.2918   $133.29   $133.30   $133.30  
  Purchase #4 $97.65   5.435%   $102.9573   $102.96   $102.95   $103.00  
  Purchase #5 $181.26   4.248%   $188.9599   $188.96   $188.95   $189.00  
  Purchase #6 $186.32   11.592%   $207.9182   $207.92   $207.90   $207.90  
  Purchase #7 $26.31   3.520%   $27.2361   $27.24   $27.25   $27.20  
  7 purchase totals = $939.41       $1,017.8541   $1,017.86   $1,017.85   $1,017.90