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 $242.62   13.337%   $274.9782   $274.98   $275.00   $275.00  
  Purchase #2 $183.35   10.453%   $202.5156   $202.52   $202.50   $202.50  
  Purchase #3 $9.39   11.367%   $10.4574   $10.46   $10.45   $10.50  
  Purchase #4 $173.55   3.686%   $179.9471   $179.95   $179.95   $179.90  
  Purchase #5 $220.65   10.571%   $243.9749   $243.97   $243.95   $244.00  
  Purchase #6 $191.61   7.204%   $205.4136   $205.41   $205.40   $205.40  
  Purchase #7 $244.25   5.628%   $257.9964   $258.00   $258.00   $258.00  
  7 purchase totals = $1,265.42       $1,375.2832   $1,375.29   $1,375.25   $1,375.30