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 $20.72   3.439%   $21.4326   $21.43   $21.45   $21.40  
  Purchase #2 $139.43   12.471%   $156.8183   $156.82   $156.80   $156.80  
  Purchase #3 $49.95   10.212%   $55.0509   $55.05   $55.05   $55.10  
  Purchase #4 $144.52   12.325%   $162.3321   $162.33   $162.35   $162.30  
  Purchase #5 $20.38   3.442%   $21.0815   $21.08   $21.10   $21.10  
  Purchase #6 $159.69   6.478%   $170.0347   $170.03   $170.05   $170.00  
  Purchase #7 $175.87   12.151%   $197.24   $197.24   $197.25   $197.20  
  7 purchase totals = $710.56       $783.9901   $783.98   $784.05   $783.90