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 $204.49   10.617%   $226.2007   $226.20   $226.20   $226.20  
  Purchase #2 $183.75   4.298%   $191.6476   $191.65   $191.65   $191.60  
  Purchase #3 $34.33   13.731%   $39.0439   $39.04   $39.05   $39.00  
  Purchase #4 $25.70   9.768%   $28.2104   $28.21   $28.20   $28.20  
  Purchase #5 $157.75   12.742%   $177.8505   $177.85   $177.85   $177.90  
  Purchase #6 $139.34   8.707%   $151.4723   $151.47   $151.45   $151.50  
  Purchase #7 $9.34   11.768%   $10.4391   $10.44   $10.45   $10.40  
  7 purchase totals = $754.70       $824.8645   $824.86   $824.85   $824.80