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 $48.57   4.694%   $50.8499   $50.85   $50.85   $50.80  
  Purchase #2 $227.51   11.153%   $252.8842   $252.88   $252.90   $252.90  
  Purchase #3 $84.29   6.735%   $89.9669   $89.97   $89.95   $90.00  
  Purchase #4 $58.97   8.440%   $63.9471   $63.95   $63.95   $63.90  
  Purchase #5 $169.57   13.612%   $192.6519   $192.65   $192.65   $192.70  
  Purchase #6 $247.79   4.847%   $259.8004   $259.80   $259.80   $259.80  
  Purchase #7 $152.53   13.470%   $173.0758   $173.08   $173.10   $173.10  
  7 purchase totals = $989.23       $1,083.1762   $1,083.18   $1,083.20   $1,083.20