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 $244.53   12.140%   $274.2159   $274.22   $274.20   $274.20  
  Purchase #2 $166.52   10.584%   $184.1445   $184.14   $184.15   $184.10  
  Purchase #3 $154.72   7.246%   $165.931   $165.93   $165.95   $165.90  
  Purchase #4 $231.97   7.700%   $249.8317   $249.83   $249.85   $249.80  
  Purchase #5 $227.88   5.218%   $239.7708   $239.77   $239.75   $239.80  
  Purchase #6 $163.58   11.144%   $181.8094   $181.81   $181.80   $181.80  
  Purchase #7 $27.41   12.349%   $30.7949   $30.79   $30.80   $30.80  
  7 purchase totals = $1,216.61       $1,326.4982   $1,326.49   $1,326.50   $1,326.40