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 $199.31   9.620%   $218.4836   $218.48   $218.50   $218.50  
  Purchase #2 $120.33   8.827%   $130.9515   $130.95   $130.95   $131.00  
  Purchase #3 $160.68   4.255%   $167.5169   $167.52   $167.50   $167.50  
  Purchase #4 $230.88   6.467%   $245.811   $245.81   $245.80   $245.80  
  Purchase #5 $115.50   8.142%   $124.904   $124.90   $124.90   $124.90  
  Purchase #6 $87.49   9.295%   $95.6222   $95.62   $95.60   $95.60  
  Purchase #7 $243.56   11.553%   $271.6985   $271.70   $271.70   $271.70  
  7 purchase totals = $1,157.75       $1,254.9877   $1,254.98   $1,254.95   $1,255.00