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 $5.40   8.514%   $5.8598   $5.86   $5.85   $5.90  
  Purchase #2 $109.29   13.132%   $123.642   $123.64   $123.65   $123.60  
  Purchase #3 $43.51   4.315%   $45.3875   $45.39   $45.40   $45.40  
  Purchase #4 $233.43   12.217%   $261.9481   $261.95   $261.95   $261.90  
  Purchase #5 $25.69   13.146%   $29.0672   $29.07   $29.05   $29.10  
  Purchase #6 $11.58   9.610%   $12.6928   $12.69   $12.70   $12.70  
  Purchase #7 $39.29   9.638%   $43.0768   $43.08   $43.10   $43.10  
  7 purchase totals = $468.19       $521.6742   $521.68   $521.70   $521.70