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 $18.37   11.565%   $20.4945   $20.49   $20.50   $20.50  
  Purchase #2 $80.67   5.178%   $84.8471   $84.85   $84.85   $84.80  
  Purchase #3 $39.58   7.396%   $42.5073   $42.51   $42.50   $42.50  
  Purchase #4 $223.80   8.849%   $243.6041   $243.60   $243.60   $243.60  
  Purchase #5 $138.52   12.870%   $156.3475   $156.35   $156.35   $156.30  
  Purchase #6 $178.58   8.323%   $193.4432   $193.44   $193.45   $193.40  
  Purchase #7 $91.82   5.679%   $97.0345   $97.03   $97.05   $97.00  
  7 purchase totals = $771.34       $838.2782   $838.27   $838.30   $838.10