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 $59.48   11.648%   $66.4082   $66.41   $66.40   $66.40  
  Purchase #2 $23.29   6.459%   $24.7943   $24.79   $24.80   $24.80  
  Purchase #3 $97.52   9.591%   $106.8731   $106.87   $106.85   $106.90  
  Purchase #4 $106.61   8.441%   $115.609   $115.61   $115.60   $115.60  
  Purchase #5 $25.46   5.784%   $26.9326   $26.93   $26.95   $26.90  
  Purchase #6 $106.60   6.270%   $113.2838   $113.28   $113.30   $113.30  
  Purchase #7 $128.78   6.169%   $136.7244   $136.72   $136.70   $136.70  
  7 purchase totals = $547.74       $590.6254   $590.61   $590.60   $590.60