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 $18.54   11.403%   $20.6541   $20.65   $20.65   $20.70  
  Purchase #2 $24.54   13.696%   $27.901   $27.90   $27.90   $27.90  
  Purchase #3 $85.53   7.457%   $91.908   $91.91   $91.90   $91.90  
  Purchase #4 $232.48   8.330%   $251.8456   $251.85   $251.85   $251.80  
  Purchase #5 $37.71   11.202%   $41.9343   $41.93   $41.95   $41.90  
  Purchase #6 $147.67   9.241%   $161.3162   $161.32   $161.30   $161.30  
  Purchase #7 $128.25   4.546%   $134.0802   $134.08   $134.10   $134.10  
  7 purchase totals = $674.72       $729.6394   $729.64   $729.65   $729.60