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 $131.57   5.179%   $138.384   $138.38   $138.40   $138.40  
  Purchase #2 $160.59   12.446%   $180.577   $180.58   $180.60   $180.60  
  Purchase #3 $99.37   3.641%   $102.9881   $102.99   $103.00   $103.00  
  Purchase #4 $82.51   7.133%   $88.3954   $88.40   $88.40   $88.40  
  Purchase #5 $40.50   6.551%   $43.1532   $43.15   $43.15   $43.20  
  Purchase #6 $9.57   4.234%   $9.9752   $9.98   $10.00   $10.00  
  Purchase #7 $250.35   9.749%   $274.7566   $274.76   $274.75   $274.80  
  7 purchase totals = $774.46       $838.2295   $838.24   $838.30   $838.40