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 $131.86   6.677%   $140.6643   $140.66   $140.65   $140.70  
  Purchase #2 $29.60   5.835%   $31.3272   $31.33   $31.35   $31.30  
  Purchase #3 $191.32   12.728%   $215.6712   $215.67   $215.65   $215.70  
  Purchase #4 $192.75   8.640%   $209.4036   $209.40   $209.40   $209.40  
  Purchase #5 $231.88   6.611%   $247.2096   $247.21   $247.20   $247.20  
  Purchase #6 $32.90   3.578%   $34.0772   $34.08   $34.10   $34.10  
  Purchase #7 $250.39   11.611%   $279.4628   $279.46   $279.45   $279.50  
  7 purchase totals = $1,060.70       $1,157.8159   $1,157.81   $1,157.80   $1,157.90