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 $7.90   4.588%   $8.2625   $8.26   $8.25   $8.30  
  Purchase #2 $7.98   5.609%   $8.4276   $8.43   $8.45   $8.40  
  Purchase #3 $250.37   9.869%   $275.079   $275.08   $275.10   $275.10  
  Purchase #4 $30.46   9.537%   $33.365   $33.37   $33.35   $33.40  
  Purchase #5 $210.35   13.479%   $238.7031   $238.70   $238.70   $238.70  
  Purchase #6 $128.93   8.839%   $140.3261   $140.33   $140.35   $140.30  
  Purchase #7 $125.61   5.811%   $132.9092   $132.91   $132.90   $132.90  
  7 purchase totals = $761.60       $837.0725   $837.08   $837.10   $837.10