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 $17.70   4.516%   $18.4993   $18.50   $18.50   $18.50  
  Purchase #2 $226.46   13.182%   $256.312   $256.31   $256.30   $256.30  
  Purchase #3 $192.89   5.776%   $204.0313   $204.03   $204.05   $204.00  
  Purchase #4 $31.98   7.131%   $34.2605   $34.26   $34.25   $34.30  
  Purchase #5 $183.84   12.285%   $206.4247   $206.42   $206.40   $206.40  
  Purchase #6 $217.42   5.821%   $230.076   $230.08   $230.10   $230.10  
  Purchase #7 $91.61   4.398%   $95.639   $95.64   $95.65   $95.60  
  7 purchase totals = $961.90       $1,045.2428   $1,045.24   $1,045.25   $1,045.20