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 $18.58   13.415%   $21.0725   $21.07   $21.05   $21.10  
  Purchase #2 $202.87   6.233%   $215.5149   $215.51   $215.50   $215.50  
  Purchase #3 $149.63   3.648%   $155.0885   $155.09   $155.10   $155.10  
  Purchase #4 $226.37   5.865%   $239.6466   $239.65   $239.65   $239.60  
  Purchase #5 $171.93   11.640%   $191.9427   $191.94   $191.95   $191.90  
  Purchase #6 $216.48   6.421%   $230.3802   $230.38   $230.40   $230.40  
  Purchase #7 $69.60   8.823%   $75.7408   $75.74   $75.75   $75.70  
  7 purchase totals = $1,055.46       $1,129.3862   $1,129.38   $1,129.40   $1,129.30