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 $250.90   5.396%   $264.4386   $264.44   $264.45   $264.40  
  Purchase #2 $32.67   11.269%   $36.3516   $36.35   $36.35   $36.40  
  Purchase #3 $70.75   13.331%   $80.1817   $80.18   $80.20   $80.20  
  Purchase #4 $219.94   9.275%   $240.3394   $240.34   $240.35   $240.30  
  Purchase #5 $12.25   13.381%   $13.8892   $13.89   $13.90   $13.90  
  Purchase #6 $174.87   5.752%   $184.9285   $184.93   $184.95   $184.90  
  Purchase #7 $188.91   9.383%   $206.6354   $206.64   $206.65   $206.60  
  7 purchase totals = $950.29       $1,026.7644   $1,026.77   $1,026.85   $1,026.70