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 $241.98   11.713%   $270.3231   $270.32   $270.30   $270.30  
  Purchase #2 $177.65   13.204%   $201.1069   $201.11   $201.10   $201.10  
  Purchase #3 $37.71   4.547%   $39.4247   $39.42   $39.40   $39.40  
  Purchase #4 $247.25   4.194%   $257.6197   $257.62   $257.60   $257.60  
  Purchase #5 $141.74   12.257%   $159.1131   $159.11   $159.10   $159.10  
  Purchase #6 $10.34   9.619%   $11.3346   $11.33   $11.35   $11.30  
  Purchase #7 $69.51   11.493%   $77.4988   $77.50   $77.50   $77.50  
  7 purchase totals = $926.18       $1,016.4209   $1,016.41   $1,016.35   $1,016.30