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 $148.92   3.254%   $153.7659   $153.77   $153.75   $153.80  
  Purchase #2 $55.48   9.606%   $60.8094   $60.81   $60.80   $60.80  
  Purchase #3 $59.55   12.358%   $66.9092   $66.91   $66.90   $66.90  
  Purchase #4 $155.35   5.813%   $164.3805   $164.38   $164.40   $164.40  
  Purchase #5 $5.84   13.476%   $6.627   $6.63   $6.65   $6.60  
  Purchase #6 $186.40   4.286%   $194.3891   $194.39   $194.40   $194.40  
  Purchase #7 $23.97   12.654%   $27.0032   $27.00   $27.00   $27.00  
  7 purchase totals = $635.51       $673.8843   $673.89   $673.90   $673.90