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 $37.43   6.828%   $39.9857   $39.99   $40.00   $40.00  
  Purchase #2 $69.82   3.115%   $71.9949   $71.99   $72.00   $72.00  
  Purchase #3 $245.54   12.544%   $276.3405   $276.34   $276.35   $276.30  
  Purchase #4 $170.64   8.690%   $185.4686   $185.47   $185.45   $185.50  
  Purchase #5 $150.29   6.117%   $159.4832   $159.48   $159.50   $159.50  
  Purchase #6 $173.84   10.380%   $191.8846   $191.88   $191.90   $191.90  
  Purchase #7 $185.52   9.765%   $203.636   $203.64   $203.65   $203.60  
  7 purchase totals = $1,033.08       $1,128.7935   $1,128.79   $1,128.85   $1,128.80