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 $46.94   12.366%   $52.7446   $52.74   $52.75   $52.70  
  Purchase #2 $8.99   7.854%   $9.6961   $9.70   $9.70   $9.70  
  Purchase #3 $216.96   10.792%   $240.3743   $240.37   $240.35   $240.40  
  Purchase #4 $156.46   8.483%   $169.7325   $169.73   $169.75   $169.70  
  Purchase #5 $112.52   9.485%   $123.1925   $123.19   $123.20   $123.20  
  Purchase #6 $177.93   7.415%   $191.1235   $191.12   $191.10   $191.10  
  Purchase #7 $14.74   12.384%   $16.5654   $16.57   $16.55   $16.60  
  7 purchase totals = $734.54       $803.4289   $803.42   $803.40   $803.40