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 $122.84   11.819%   $137.3585   $137.36   $137.35   $137.40  
  Purchase #2 $32.55   4.589%   $34.0437   $34.04   $34.05   $34.00  
  Purchase #3 $103.85   6.791%   $110.9025   $110.90   $110.90   $110.90  
  Purchase #4 $109.31   3.388%   $113.0134   $113.01   $113.00   $113.00  
  Purchase #5 $6.25   9.636%   $6.8523   $6.85   $6.85   $6.90  
  Purchase #6 $71.97   8.153%   $77.8377   $77.84   $77.85   $77.80  
  Purchase #7 $8.77   12.283%   $9.8472   $9.85   $9.85   $9.80  
  7 purchase totals = $455.54       $489.8553   $489.85   $489.85   $489.80