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 $134.25   3.461%   $138.8964   $138.90   $138.90   $138.90  
  Purchase #2 $122.68   9.289%   $134.0757   $134.08   $134.10   $134.10  
  Purchase #3 $203.99   11.500%   $227.4489   $227.45   $227.45   $227.40  
  Purchase #4 $5.60   3.333%   $5.7866   $5.79   $5.80   $5.80  
  Purchase #5 $189.43   8.440%   $205.4179   $205.42   $205.40   $205.40  
  Purchase #6 $19.86   10.812%   $22.0073   $22.01   $22.00   $22.00  
  Purchase #7 $92.76   5.267%   $97.6457   $97.65   $97.65   $97.60  
  7 purchase totals = $768.57       $831.2785   $831.30   $831.30   $831.20