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 $124.92   9.142%   $136.3402   $136.34   $136.35   $136.30  
  Purchase #2 $168.65   7.339%   $181.0272   $181.03   $181.05   $181.00  
  Purchase #3 $187.92   9.561%   $205.887   $205.89   $205.90   $205.90  
  Purchase #4 $119.72   5.210%   $125.9574   $125.96   $125.95   $126.00  
  Purchase #5 $146.52   11.379%   $163.1925   $163.19   $163.20   $163.20  
  Purchase #6 $149.30   10.749%   $165.3483   $165.35   $165.35   $165.30  
  Purchase #7 $24.83   3.256%   $25.6385   $25.64   $25.65   $25.60  
  7 purchase totals = $921.86       $1,003.3911   $1,003.40   $1,003.45   $1,003.30