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 $124.88   9.232%   $136.4089   $136.41   $136.40   $136.40  
  Purchase #2 $189.58   3.825%   $196.8314   $196.83   $196.85   $196.80  
  Purchase #3 $169.30   6.488%   $180.2842   $180.28   $180.30   $180.30  
  Purchase #4 $25.76   12.406%   $28.9558   $28.96   $28.95   $29.00  
  Purchase #5 $94.35   12.831%   $106.456   $106.46   $106.45   $106.50  
  Purchase #6 $63.54   3.374%   $65.6838   $65.68   $65.70   $65.70  
  Purchase #7 $231.81   12.226%   $260.1511   $260.15   $260.15   $260.20  
  7 purchase totals = $899.22       $974.7712   $974.77   $974.80   $974.90