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 $7.32   10.798%   $8.1104   $8.11   $8.10   $8.10  
  Purchase #2 $18.88   8.132%   $20.4153   $20.42   $20.40   $20.40  
  Purchase #3 $181.25   4.590%   $189.5694   $189.57   $189.55   $189.60  
  Purchase #4 $121.42   5.685%   $128.3227   $128.32   $128.30   $128.30  
  Purchase #5 $37.25   7.594%   $40.0788   $40.08   $40.10   $40.10  
  Purchase #6 $160.27   7.767%   $172.7182   $172.72   $172.70   $172.70  
  Purchase #7 $183.56   5.581%   $193.8045   $193.80   $193.80   $193.80  
  7 purchase totals = $709.95       $753.0193   $753.02   $752.95   $753.00