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 $130.75   5.490%   $137.9282   $137.93   $137.95   $137.90  
  Purchase #2 $123.44   12.417%   $138.7675   $138.77   $138.75   $138.80  
  Purchase #3 $93.41   9.546%   $102.3269   $102.33   $102.35   $102.30  
  Purchase #4 $151.71   3.751%   $157.4006   $157.40   $157.40   $157.40  
  Purchase #5 $231.38   8.518%   $251.0889   $251.09   $251.10   $251.10  
  Purchase #6 $43.79   12.217%   $49.1398   $49.14   $49.15   $49.10  
  Purchase #7 $84.26   9.426%   $92.2023   $92.20   $92.20   $92.20  
  7 purchase totals = $858.74       $928.8542   $928.86   $928.90   $928.80