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 $242.53   11.755%   $271.0394   $271.04   $271.05   $271.00  
  Purchase #2 $24.51   4.631%   $25.6451   $25.65   $25.65   $25.60  
  Purchase #3 $198.84   3.575%   $205.9485   $205.95   $205.95   $205.90  
  Purchase #4 $96.38   3.490%   $99.7437   $99.74   $99.75   $99.70  
  Purchase #5 $116.87   5.497%   $123.2943   $123.29   $123.30   $123.30  
  Purchase #6 $72.58   6.332%   $77.1758   $77.18   $77.20   $77.20  
  Purchase #7 $122.97   13.665%   $139.7739   $139.77   $139.75   $139.80  
  7 purchase totals = $874.68       $942.6207   $942.62   $942.65   $942.50