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 $28.62   12.789%   $32.2802   $32.28   $32.30   $32.30  
  Purchase #2 $175.30   9.621%   $192.1656   $192.17   $192.15   $192.20  
  Purchase #3 $24.29   6.787%   $25.9386   $25.94   $25.95   $25.90  
  Purchase #4 $130.52   6.427%   $138.9085   $138.91   $138.90   $138.90  
  Purchase #5 $220.55   4.546%   $230.5762   $230.58   $230.60   $230.60  
  Purchase #6 $171.96   5.730%   $181.8133   $181.81   $181.80   $181.80  
  Purchase #7 $236.47   4.422%   $246.9267   $246.93   $246.95   $246.90  
  7 purchase totals = $987.71       $1,048.6091   $1,048.62   $1,048.65   $1,048.60